Influence of microbe-associated parthenogenesis on the fecundity ofTrichogramma deion andT. pretiosum

Microbe-associated parthenogenesis (thelytoky) has been discovered in nineTrichogramma species, parasitoids of mainly lepidopteran eggs. Parthenogenetic and bisexual conspecifics co-occur in many field populations. As an initial step to understand the dynamics of these two reproductive strategies we studied the effect of microbe-associated parthenogenesis on fecundity. The fecundity of two parthenogenetic isofemale lines ofT. pretiosum and one ofT. deion was compared with bisexual lines derived from them by antibiotic treatment. In all three cases parthenogenetic females were less fecund over their lifetime than bisexual females. Also, parthenogenetic females produced fewer daughters in two cases and in one case a similar number of daughters as their respective bisexual counterparts. The lack of mating and insemination was excluded as an explanation for the reduced fecundity of parthenogenetic females, because mated and virgin parthenogenetic females produce the same number of offspring. Antibiotic treatment can also be excluded because females of field-collected bisexual line treated with antibiotics produced the same number of offspring as untreated females. The reduced fecundity of parthenogenetic females was caused by a lower number of eggs being laid rather than by a greater developmental mortality. Parthenogenetic females produced less daughters than bisexual females when host availability was not limiting, but when host availability was severely limited, parthenogenetic females produced more daughters than the bisexual females.     

Superparasitism strategies by a native and an exotic parasitoid species attacking the Mexican fruit fly,Anastrepha ludens (Diptera: Tephritidae)

Superparasitism refers to the action of parasitoids ovipositing eggs in hosts that are already parasitized; this inevitably results in the elimination of supernumerary larvae in solitary parasitoids. Here, we investigated superparasitism performed by two species of solitary parasitoids on the larvae of Anastrepha ludens (Loew; Diptera: Tephritidae): a native species, Doryctobracon crawfordi (Viereck; Hymenoptera: Braconidae), and an exotic species, Diachasmimorpha tryoni (Cameron; Hymenoptera: Braconidae). Tests were conducted under laboratory conditions evaluating the behaviour of females acting alone (self-superparasitism) or in groups (conspecific superparasitism). Parasitism strategies were different between these two species. In D. crawfordi, the number of first instar larvae found in each dissected host pupa was never greater than two, regardless of the number of oviposition scars observed per pupa. In contrast, there was a positive correlation between the number of oviposition scars and the number of first instar larvae in D. tryoni. The survival and fecundity of D. crawfordi females emerging from pupae with one scar was higher than in females emerging from pupae with more scars. In D. tryoni, the number of oviposition scars did not show deleterious effects on life history traits and was positively correlated with the proportion of emerging females. An understanding of the superparasitism strategy adopted by parasitoid species could be of great interest to augmentative biological control programmes because the mass rearing of natural enemies could be negatively or positively affected by this condition.     

HOST IMMUNE STATUS DETERMINES SEXUALITY IN A PARASITIC NEMATODE

We examine the hypothesis that sexual reproduction by parasites is an adaptation to counter the somatic evolution of vertebrate immune responses. This is analogous to the idea that antagonistic coevolution between hosts and their parasites maintains sexual reproduction in host populations. Strongyloides ratti is a parasitic nematode of rats. It can have a direct life cycle, with clonal larvae of the wholly parthenogenetic parasites becoming infective, or an indirect life cycle, with clonal larvae developing into free-living dioecious adults. These free-living adults produce infective larvae by conventional meiosis and syngamy. The occurrence of the sexual cycle is determined by both environmental and genetic factors. By experimentally manipulating host immune status using hypothymic mutants, corticosteroids, whole-body γ-irradiation and previous exposure to S. ratti, we show that larvae from hosts that have acquired immune protection are more likely to develop into sexual adults. This effect is independent of the method of manipulation, larval density, and the number of days postinfection. This immune-determined sexuality is consistent with the idea that sexual reproduction by parasites is adaptive in the face of specific immunity, an idea which, if true, has clinical and epidemiological consequences.     

Geographic parthenogenesis and the common tea‐tree stick insect of New Zealand

Worldwide, parthenogenetic reproduction has evolved many times in the stick insects (Phasmatidae). Many parthenogenetic stick insects show the distribution pattern known as geographic parthenogenesis, in that they occupy habitats that are at higher altitude or latitude compared with their sexual relatives. Although it is often assumed that, in the short term, parthenogenetic populations will have a reproductive advantage over sexual populations; this is not necessarily the case. We present data on the distribution and evolutionary relationships of sexual and asexual populations of the New Zealand stick insect, Clitarchus hookeri. Males are common in the northern half of the species’ range but rare or absent elsewhere, and we found that most C. hookeri from putative‐parthenogenetic populations share a common ancestor. Female stick insects from bisexual populations of Clitarchus hookeri are capable of parthenogenetic reproduction, but those insects from putative‐parthenogenetic populations produced few offspring via sexual reproduction when males were available. We found similar fertility (hatching success) in mated and virgin females. Mated females produce equal numbers of male and female offspring, with most hatching about 9–16 weeks after laying. In contrast, most eggs from unmated females took longer to hatch (21–23 weeks), and most offspring were female. It appears that all C. hookeri females are capable of parthenogenetic reproduction, and thus could benefit from the numerical advantage this yields. Nevertheless, our phylogeographic evidence shows that the majority of all‐female populations over a wide geographic area originate from a single loss of sexual reproduction.     

Ecological Implications of Clonal Diversity in Parthenogenetic Morphospecies

Most parthenogenetic animal taxa which have been investigatedelectrophoretically, cytologically, or with tissue graftingtechniques are clonally diverse. I have examined data on multiclonalparthenogenetic populations using ecological diversity measuresto elucidate patterns of clonal coexistence. Analysis of a discretepopulation cage experiment on clones of Drosophila mercatorumrevealed monotonic decay of clonal diversity and evenness; however,in a continuous generation cage, clonal diversity appeared tostabilize. Clonal diversity and evenness fluctuated widely overtime in several multiclonal populations of Daphnia magna althoughno clonal extinction was observed. There were few spatial trendsin clonal diversity and evenness within parthenogenetic taxa.It is suggested that the degree of clonal differentiation, determinedby the mode of clonal origin, is important in determining whetheror not selection occurs among sympatric clones     

POLYPHYLETIC ORIGINS OF ASEXUALITY IN DAPHNIA PULEX. I. BREEDING-SYSTEM VARIATION AND LEVELS OF CLONAL DIVERSITY

There is growing evidence that transitions from sexual to asexual reproduction are often provoked by internal genetic factors rather than extrinsic selection pressures. In the cladoceran crustacean Daphnia pulex, the shift to asexuality has been linked to sex-limited meiosis suppression. Most populations of this species reproduce by obligate parthenogenesis, but cyclically parthenogenetic populations persist in the southern portion of its range. The meiosis-suppressor model predicts that asexuality in D. pulex has polyphyletic origins and that the coexistence of cyclically parthenogenetic lines with male-producing obligately asexual clones should be unstable. For the present study, we examined the genotypic structure of D. pulex populations from a region in which there is an abrupt microgeographical shift in breeding system. Populations in Michigan largely reproduce by cyclic parthenogenesis, while those in Ontario are obligately asexual. Allozyme studies on 77 populations from this area revealed 50 obligately asexual clones, divisible into two groups: one derived from a single parent species and the other derived via interspecific hybridization. Although nearly 50% of the clones retained male production, there was, as predicted, no evidence of coexistence between cyclically parthenogenetic populations and male-producing obligately asexual clones. The survey did, however, reveal a low incidence of cyclically parthenogenetic populations in Ontario. The high genotypic diversity of these populations suggests that they are not only resistant to meiosis suppression, but able to rework genetic variation gained from asexual clones into a sexual breeding system.     

Fecundity,reproductive effort,and pupal size in the profundal midgeChironomus cuccini (Diptera: Chironomidae)

Fecundity ofChironomus cucini increased linearly across a three-fold range of female pupal biomass. Females from a lake producing small pupae (Crystal Lake) had an average of 303 primary follicles, while larger pupae from Trout Lake had an average fecundity of 582. Pupae produced in Crystal Lake from larvae that received experimental food supplements were intermediate in size and fecundity. Pupal biomass explained 76% of the variation in fecundity. No difference in egg size was detected between pupae from the two lakes, and an average value of 2.1 g egg^–1 was used to calculate the total biomass allocated to eggs in each of 79 females dissected. This measure of reproductive effort scaled isometrically with pupal mass. On average, females allocated 48% of their biomass to eggs and 52% to somatic tissues. A comparison of average male biomass to female somatic biomass across 12 populations indicated that the biomass difference between the sexes is a biased predictor of fecundity.     

Females from resting eggs and parthenogenetic eggs in the rotifer Brachionus calyciflorus: lipid droplets, starvation resistance and reproduction

1. In the heterogonic life cycle of monogonont rotifers, amictic (female‐producing) females develop from two types of eggs: fertilised resting (diapausing) eggs and parthenogenetic subitaneous eggs. Females hatched from resting eggs initiate clonal populations by female parthenogenesis and are called stem females. This study compares females from resting and parthenogenetic eggs that were produced under identical culture conditions and were of similar birth order. 2. Newborn stem females had many more lipid droplets in their tissues than similar‐sized, newborn females from parthenogenetic eggs. When neonates were stained with Nile Red and viewed under epifluorescent illumination, these droplets were shown to be sites of neutral‐lipid storage products. 3. Stem females had no posterolateral spines and short anterior spines, while their mothers and offspring in subsequent, parthenogenetic generations typically had long posterolateral spines and elongated anterior spines. 4. Newborn stem females survived starvation significantly longer than newborn females from parthenogenetic eggs. 5. When females from resting and parthenogenetic eggs were cultured from birth to death at a high food concentration, the reproductive potential (r day^?1) of the stem females was significantly higher (0.82–0.88 versus 0.70), primarily because of egg production at an earlier age. The mean lifetime fecundity (R_o) of stem females was significantly greater than that of females from parthenogenetic eggs. 6. Extensive lipid reserves should increase the ability of stem females to colonise new habitats. Firstly, compared with females from parthenogenetic eggs, stem females are more likely to experience starvation or food limitation. Resting eggs hatch in response to physical and chemical factors that are not directly related to food availability, and from sediments that may be far from food‐rich surface waters. Secondly, when food is abundant, stem females have a greater reproductive potential.     

Sexual reproduction of Daphnia pulex in a temporary habitat

Species of Daphnia (Crustacea: Cladocera) typically reproduce by cyclical parthenogenesis, in which a period of all-female parthenogenetic reproduction is followed by sexual reproduction. Sex in Daphnia is determined by the environment, with factors such as temperature, photoperiod and crowding stimulating the production of males and sexual females. Previous studies on Daphnia pulex from temporary pond habitats demonstrated the coexistence of male-producing and non-male-producing (NMP) females, as determined under crowding in the laboratory. A strong genetic component to this sex allocation variation suggested that sex expression in D. pulex is better described as a result of genotype-environment interaction. The present study examined the switch from parthenogenetic to sexual reproduction in two temporary-pond populations of D. pulex. Both populations showed a very early investment in sexual reproduction, independent of population density, by producing males very soon after the populations were reestablished from resting eggs in the early spring. Approximately 40% of the initial broods were male. Additional evidence for gender specialization was obtained by observing the sex of two or three successive broods for 85 individual females. Fifty-eight females produced successive broods of females, 13 females produced successive broods of males and 14 females produced successive broods which included both male and female broods. Females that produced successive female broods under natural conditions included a higher frequency of NMP females compared to a random sample of females, confirming the existence of NMP females. Sexual females were observed in both populations after the first appearence of males, suggesting that the presence of males may stimulate the production of sexual females. For D. pulex populations in a temporary environment, there appears to be an increased emphasis on sexual reproduction and a decreased influence of the environment on sex determination, compared to Daphnia populations in more permanent habitats. Received: 19 February 1996 / Accepted: 20 January 1997     

Comparisons of life-history traits between clonal and sexual fish (Poeciliopsis:Poeciliidae) raised in monoculture and mixed treatments

Summary Clonal and sexual co-existence is common in a number of vertebrate taxa, even though the cost of sex makes such co-existence theoretically unlikely. The frozen niche-variation (FNV) model explains this co-existence on the basis of differences in overall niche breadth and competition between clones and sexuals. In the present study I examined two predictions of the FNV model. First, I examined the prediction that genetically variable populations have higher relative fitness when compared with monoclonal populations by comparing the performances of clonal and outcrossed sexual strains ofPoeciliopsis in monocultures at two densities. The prediction of increased overall productivity for the sexuals was verified, with net reproductive rates for the sexuals being between two and four times as high as the clones. Second, I tested the prediction that derived clones will successfully compete with their sexual progenitor(s) in the narrow range to which the clones are adapted, while the sexuals should co-exist because of their ability to use a wider range of resources than any single clone. I examined this prediction by comparing performance variables (e.g. growth, fecundity and survival) of each strain in pure culture with their partitioned performance from the mixed treatments. Clonal performance increased in mixtures compared to monocultures, as expected. However, the expectation that the sexual's performance would be less affected by mixtures than the clones' performance, was not met. The sexuals had reduced growth and fecundity on a par with the increase in both variables in the clones. Therefore, support for the FNV model was mixed. Although the performance in monocultures suggests that the sexuals have a wider niche breadth than the clones, performances in mixtures do not indicate such a relationship. Switching of behaviours or resource-use patterns between mixed and pure cultures may have caused the equivocal results.     

EFFECTS OF PARTHENOGENESIS AND GEOGRAPHIC ISOLATION ON FEMALE SEXUAL TRAITS IN A PARASITOID WASP

Population divergence in sexual traits is affected by different selection pressures, depending on the mode of reproduction. In allopatric sexual populations, aspects of sexual behavior may diverge due to sexual selection. In parthenogenetic populations, loss‐of‐function mutations in genes involved in sexual functionality may be selectively neutral or favored by selection. We assess to what extent these processes have contributed to divergence in female sexual traits in the parasitoid wasp Leptopilina clavipes in which some populations are infected with parthenogenesis‐inducing Wolbachia bacteria. We find evidence consistent with both hypotheses. Both arrhenotokous males and males derived from thelytokous strains preferred to court females from their own population. This suggests that these populations had already evolved population‐specific mating preferences when the latter became parthenogenetic. Thelytokous females did not store sperm efficiently and fertilized very few of their eggs. The nonfertility of thelytokous females was due to mutations in the wasp genome, which must be an effect of mutation accumulation under thelytoky. Divergence in female sexual traits of these two allopatric populations has thus been molded by different forces: independent male/female coevolution while both populations were still sexual, followed by female‐only evolution after one population switched to parthenogenesis.     

The fitness effects of delayed switching to sex in a facultatively asexual insect

Facultative reproductive strategies that incorporate both sexual and parthenogenetic reproduction should be optimal, yet are rarely observed in animals. Resolving this paradox requires an understanding of the economics of facultative asexuality. Recent work suggests that switching from parthenogenesis to sex can be costly and that females can resist mating to avoid switching. However, it remains unclear whether these costs and resistance behaviors are dependent on female age. We addressed these questions in the Cyclone Larry stick insect, Sipyloidea larryi, by pairing females with males (or with females as a control) in early life prior to the start of parthenogenetic reproduction, or in mid‐ or late life after a period of parthenogenetic oviposition. Young females were receptive to mating even though mating in early life caused reduced fecundity. Female resistance to mating increased with age, but reproductive switching in mid‐ or late life did not negatively affect female survival or offspring performance. Overall, mating enhanced female fitness because fertilized eggs had higher hatching success and resulted in more adult offspring than parthenogenetic eggs. However, female fecundity and offspring viability were also enhanced in females paired with other females, suggesting a socially mediated maternal effect. Our results provide little evidence that switching from parthenogenesis to sex at any age is costly for S. larryi females. However, age‐dependent effects of switching on some fitness components and female resistance behaviors suggest the possibility of context‐dependent effects that may only be apparent in natural populations.     

Reproductive Consequences of Clonal Growth in Stenocereus eruca, a Rare Clonal Cactus of the Sonoran Desert

Stenocereus eruca is a prostrated, self-incompatible cactus endemic to the Sonoran Desert that regenerates primarily through clonal propagation. Clonal growth is expected to affect mate availability by influencing the number and spatial distribution of mating types. In this paper we examine the role of clonal growth on female fecundity through a series of pollination experiments in a population of S. eruca. We set up a pollen supplementation experiment using five distance treatments with pollen collected at 1, 10, 100, 1000 and 25000 m from receptor flowers during the years 2001 and 2002 and evaluated genetic sifmilarities between pairs of receptor-donor ramets through RAPD markers. Our data on fruit set, number of seeds/fruit, germination and overall fecundity revealed that S. eruca show a significant reduction in female fecundity when pollination occurs between ramets located at short distances (1 and 10 m), while genetic data showed high levels of similarity at those distances. The reduction in female fecundity is apparently a consequence of geitonogamy and inbreeding depression. Our data suggest that clonal growth and geitonogamy are likely to be partially responsible for the low levels of sexual reproduction and seedling recruitment observed in populations of S. eruca. Co-ordinating editor: H. Kudoh     

Diverse,endemic and polyphyletic clones in mixed populations of a freshwater snail (Potamopyrgus antipodarum)

Understanding the source and diversity of clones is necessary to resolve the complicated issues surrounding the apparent evolutionary stability of sexual reproduction. The source of clones is important because present theory is based on an “all else equal” assumption, which is predicated on the idea that clonal mutants are derived from and compete with local sexual populations. Clonal diversity is important because it reduces the advantage of sexual reproduction under either soft selection (the Tangled Bank Hypothesis) or under strict frequency-dependent selection (the Red Queen Hypothesis). In the present study, protein electrophoresis was used to determine the source and diversity of clones in a freshwater snail (Potamopyrgus antipodarum) in four glacial lakes in which sexual and clonal females were thought to coexist. The results showed (1) that the populations were mixtures of diploid sexual and triploid asexual individuals, (2) that genotypic diversity of clonal populations is very high in all four lakes (but lower than in the sympatric sexual populations), and (3) that the clones are polyphyletically derived from their sympatric sexual populations. Consequently, repeated mutation to parthenogenetic reproduction since the Pleistocene has introduced a different and diverse set of clones in all four lakes. Such diversity may provide a challenge for the ecological theories of sex that rely on frequency-dependent selection.     

Resource Availability as Driving Factor of the Reproductive Mode in Soil Microarthropods (Acari,Oribatida)

The availability of high quality resources is an important factor driving community structure and reproductive mode of animals. Parthenogenetic reproduction prevails when resources are available in excess, whereas sexuality correlates with resource shortage. We investigated the effect of resource availability on the community structure of oribatid mites in a laboratory experiment. Availability of food resources was increased by addition of glucose to leaf litter and reduced by leaching of nutrients from leaf litter. Experimental systems were incubated at three different temperatures to establish different regimes of resource exploitation. Community structure of oribatids and numbers of eggs per female were measured over a period of ten months. We expected the density of oribatid mites to decline in the reduced litter quality treatment but to increase in the glucose treatment. Both effects were assumed to be more pronounced at higher temperatures. We hypothesized sexual species to be less affected than parthenogenetic species by reduced resource quality due to higher genetic diversity allowing more efficient exploitation of limited resources, but to be outnumbered by parthenogenetic species in case of resource addition due to faster reproduction. In contrast to our hypotheses, both sexual and parthenogenetic oribatid mite species responded similarly with their densities declining uniformly during incubation. The parthenogenetic Brachychthoniidae and Tectocepheus dominated early in the experiment but were replaced later by parthenogenetic Desmonomata and Rhysotritia. In parthenogenetic species the number of eggs per female increased during the experiment while the number of eggs in sexual females remained constant or decreased slightly; in general, egg numbers were higher in sexual than in parthenogenetic species. The results indicate that for sustaining oribatid mite populations other resources than litter and associated saprotrophic microorganisms are needed. They also indicate that there are two groups of parthenogenetically reproducing species: exploiters of easily available resources and consumers of leaf litter associated resources.     

Endogenous regulation of environmentally induced sexuality in a rotifer: a multigenerational parental effect induced by fertilisation

SUMMARY 1. Sexual reproduction in the heterogonic life cycle of many rotifers occurs when amictic females, which produce diploid eggs developing parthenogenetically into females, are environmentally induced to produce mictic females. Mictic females produce haploid eggs which develop parthenogenetically into males or, if fertilised, into resting eggs – encysted embryos which develop into amictic females after an obligatory diapause. 2. A Florida strain of Brachionus calyciflorus was used to test the prediction that amictic females hatching from resting eggs (Generation 1), and those from the next few parthenogenetic generations, have a lower propensity to produce mictic daughters in response to crowding than those from later parthenogenetic generations. In 10 replicate clones, populations initiated by amictic females from generations 1, 5, 8, 12 and 18 were exposed to a standardised crowding stimulus, and the proportion of mictic females in the populations was determined. These proportions varied significantly across generations and clones. They were very low in the early generations and gradually increased to a mean of about 0.5 at Generation 12. 3. The mechanism for the transgenerational plasticity in response to crowding is not known. One possibility is that resting eggs contain an agent from their fertilised mictic mother's yolk gland that prevents development into mictic females and is transmitted in increasingly low concentrations through successive parthenogenetic generations of amictic females. 4. This parental effect may contribute to clonal fitness by ensuring that a clone developing from a resting egg will attain a higher population size through female parthenogenesis before maximising its commitment to sexual reproduction, even in the presence of a crowding stimulus from a high population density of other clones. Therefore, the number of resting eggs to which a clone contributes its genes should be maximised. 5. The clonal variation in propensity to produce mictic females in this strain indicates genetic variation in the trade‐off between maximising population growth via female parthenogenesis and increasing the probability of producing at least some resting eggs before local extinction from the plankton.     

Reproductive attributes of the eulophid Oomyzus sokolowskii,a biological control agent of diamondback moth,Plutella xylostella (Lepidoptera: Plutellidae)

Oomyzus sokolowskii (Kurdjumov) is a gregarious larval-pupal parasitoid of the diamondback moth, Plutella xylostella L. Under laboratory conditions, we explored the reproductive attributes including the functional response of O. sokolowskii and the impact of mutual interference among female parasitoids on progeny production and progeny sex ratio. In the functional response experiment, groups of 1–50 late instar larvae were exposed to groups of five female parasitoids for 48 h. Both Holling's disc equation (type II response) and Williams and Martinez's model (type III response) fit significantly better than the null-model assuming density-independent parasitation efficiency. Williams and Martinez's model was only marginally better than Holling's disc equation which explained 82% of the variation in the number of parasitized hosts. According to Holling's disc equation, the estimated maximum number of hosts parasitised during 48 h was 17 per group of five parasitoids, the estimated attack efficiency of a single female parasitoid (a) was 0.0024 larvae per hour, and the estimated handling time of a single female (T _h) was 14.4 h per host. In the mutual interference experiment 50 host larvae were exposed to different numbers of female parasitoids (P=5, 10, 20, 30, 40 and 50) for 48 h. With increasing numbers of conspecifics, the proportion of male progeny increased and total number of progeny per parasitoid decreased, yielding an estimated mutual interference constant (m) (±SE) of 0.95±0.14. The total number of wasps emerging from individual host pupae increased with increasing number of conspecifics. The age-specific fecundity of O. sokolowskii was investigated by providing 10 host larvae daily to individual female parasitoids. After 3 days of oviposition, 90% of the females were still alive and had achieved 76.4% of their lifetime fecundity. The proportion of female progeny decreased with female age. No relationship was found between female longevity and lifetime fecundity. The results suggest that parasitoid and host densities influence the progeny production and sex allocation strategy of O. sokolowskii. The information obtained from this study would help to develop a mass-rearing protocol for O. sokolowskii.     

Male mate choice in the gynogenetic–sexual complex of crucian carp,<Emphasis Type="Italic"> Carassius auratus</Emphasis>

The asexual all-female Japanese crucian carp, Carassius auratus langsdorfii (Teleostei: Cypriniformes), reproduces gynogenetically, relying on the sperm of males of the sexual "host," C. auratus subspp. Theoretically, frequency-dependent mating preference of males to conspecific females can lead to the coexistence of asexual and sexual fish, if all else is equal. Our specific questions are whether males prefer conspecific females over asexual females and whether individuals show dominance hierarchies that potentially cause frequency-dependent mating preference. In an individual choice experiment, a tank was partitioned into three compartments with the middle one for a single male and the two outer ones for a sexual and an asexual female. The males of C. auratus bürgeri demonstrated a significant preference for ovulated conspecific females over ovulated asexual females. In contrast, in a group mating experiment, a single experimental tank included two males, a sexual female, and an asexual female together, and males chased and mated with both asexual and sexual females equally. Male mate preference was weak in group mating, which is typical in natural populations. Males and females of crucian carp showed no apparent agonistic behavior to each other in the group mating experiment. This is different from other gynogenetic complexes with the dominance hierarchy of males showing strong frequency-dependent mating preference (e.g., Poeciliopsis). We conclude that male mate preference is unlikely to be a strong frequency-dependent force maintaining the coexistence of asexual–sexual complexes of Japanese crucian carp. Received in revised form: 5 February 2001 Electronic Publication     

Parthenogenetic female populations in the brown alga Scytosiphon lomentaria (Scytosiphonaceae,Ectocarpales): decay of a sexual trait and acquisition of asexual traits

In isogamous brown algae, the sexuality of populations needs to be tested by laboratory crossing experiments, as the sexes of gametophytes are morphologically indistinguishable. In some cases, gamete fusion is not observed and the precise reproductive mode of the populations is unknown. In the isogamous brown alga Scytosiphon lomentaria in Japan, both asexual (gamete fusion is unobservable) and sexual populations (gamete fusion is observable) have been reported. In order to elucidate the reproductive mode of asexual populations in this species, we used PCR‐based sex markers to investigate the sex ratio of three asexual and two sexual field populations. The markers indicated that the asexual populations consisted only of female individuals, whereas sexual populations are composed of both males and females. In culture, female gametes of most strains from asexual populations were able to fuse with male gametes; however, they had little to no detectable sexual pheromones, significantly larger cell sizes, and more rapid parthenogenetic development compared to female/male gametes from sexual populations. Investigations of sporophytic stages in the field indicated that alternation of gametophytic and parthenosporophytic stages occur in an asexual population. These results indicate that the S. lomentaria asexual populations are female populations that lack sexual reproduction and reproduce parthenogenetically. It is likely that females in the asexual populations have reduced a sexual trait (pheromone production) and have acquired asexual traits (larger gamete sizes and rapid parthenogenetic development).     

Age-specific bioassays and fecundity of Phthorimaea operculella (Lepidoptera: Gelechiidae) reared on cry1Ac -transgenic potato plants

Variation in the susceptibility of lepidopterous pest larvae of different ages to transgenic crops and the potential for survivors to reproduce could have important consequences for the development of resistance in such pests. Experiments were undertaken in the laboratory to determine if larvae of the potato tuber moth, Phthorimaea operculella, of different ages (0 (< 1 day old), 3, 5, 7 days) varied in their susceptibility to cry1Ac9–transgenic potato (Solanum tuberosum) foliage grown in the glasshouse or field. The survival and fecundity of larvae reared on transgenic tubers was also determined in the laboratory. There were no apparent differences in susceptibility of larvae of different ages to transgenic foliage. Larvae fed glasshouse or field‐grown non‐transgenic foliage had significantly larger relative growth indices and more larvae pupated, than those fed transgenic foliage, regardless of larval age. Eggs from a laboratory colony were placed on transgenic or non‐transgenic tubers to measure survival and fecundity. Between 6% and 15% of eggs placed on transgenic tubers developed into pupae for three of the four transgenic potato lines tested. On one transgenic line, only six adults emerged from 1300 eggs. In contrast, between 71% and 97% of the eggs placed on non‐transgenic tubers developed into pupae. Male and female pupae from transgenic lines weighed less than those from non‐transgenic lines. The fecundity of females from two of four transgenic lines was lower than from the non‐transgenic parent cultivar. Although larvae of different ages did not exhibit any overall age‐dependent pattern of increasing or decreasing susceptibility to transgenic foliage of glasshouse or field‐grown plants, the ability of larvae to survive and reproduce on transgenic tubers suggests this pest has the ability to evolve resistance to the transgenic plants used in the present study.