Parthenogenesis is rare in the reproduction of a sexual field population of the isogamous brown alga Scytosiphon (Scytosiphonaceae,Ectocarpales)

Parthenogenetic development of unfused gametes is commonly observed in laboratory cultures among various brown algal taxa. There is, however, little information on the contribution of parthenogenesis to the reproduction of field populations. In this study, we investigated whether parthenogenesis is present in a sexual population of the isogamous brown alga Scytosiphon with a 1:1 sex ratio. In culture, both female and male gametes showed higher mortality and slower development compared to zygotes. More than 90% of surviving partheno‐germlings formed parthenosporophytes irrespective of the culture conditions tested. Therefore, if parthenogenesis occurs in the field, most unfused gametes are expected to form parthenosporophytes. Contrary to this expectation, parthenosporophytes were rare in the field population. We collected 126 sporophytic thalli and isolated and cultured a unilocular sporangium from each of them. We confirmed that cultures of 120 unilocular sporangia produced both female and male gametophytes by the observation of zygotes or amplification of PCR‐based sex markers indicating that these sporangia originated from zygotic sporophytes. Only females were detected in cultures from two sporangia and only males from four sporangia suggesting that these sporangia originated from parthenosporophytes. In the Scytosiphon population, although parthenogenesis is observable in culture, our results demonstrate that the contribution of parthenogenesis to reproduction is small (≤4.8%) compared to sexual reproduction. Unfused gametes may not survive to form mature parthenosporophytes in significant numbers in the field partly due to their higher mortality and slower development compared from zygotes.     

Variation in Population and Life History Traits of the American Eel, Anguilla rostrata, in Four Rivers in Maine

We examined population traits of yellow American eels from nine sites with similar habitat characteristics in each of four rivers in Maine, U.S.A. Migrating silver eels were also collected to compare sex ratio, age and size at migration among the four rivers. Population density and biomass were not significantly different among rivers with mean ranges of 8.4–21.8 eels 100m^–2and 380–1485gm^–2. Pairwise comparisons of the slopes of weight–length relationships of log transformed data (pooled data: intercept = –6.007, slope = 3.094, r²= 0.99, and n = 3116) revealed no significant differences among rivers. Length–age relationships (pooled data: intercept = 87.826, slope = 23.444, r²= 0.76, and n = 2325) also showed no statistically significant pairwise differences in slopes among rivers. In all rivers, sexual differentiation was complete by 270mm total length and age eleven. The sex ratios of migrating silver eels were not correlated with yellow eel sex ratios among the four rivers. Mean age at migration among the four rivers was significantly different for males only, with a range of 1.3years. Both sexes had some significant differences in size at migration among rivers, but the biological importance of the differences is tenuous (male range: 15mm, female range: 36mm). The yellow and silver eel population traits from these four rivers showed little variation when riverine habitat was isolated. Variations in traits appeared to be greater when eels from non-riverine habitats may have been present.     

Reproductive mode and population genetic structure of the cereal aphid Sitobion avenae studied using phenotypic and microsatellite markers

As French populations of the aphid Sitobion avenae exhibit a range of reproductive modes, this species provides a good opportunity for studying the evolution of breeding system variation. The present analysis combined ecological and genetic investigations into the spatial distribution of variation in reproductive mode. Reproductive mode was characterized in 277 lineages of S. avenae from France, and these aphids were scored for five microsatellite loci. The analyses revealed strong geographical partitioning of breeding systems, with obligate asexuals mostly restricted to the south of France, while lineages producing sexual forms were more common in the north. Contrary to what might be anticipated for organisms with frequent parthenogenesis, there was substantial genic and genotypic diversity, even in the obligately asexual lineages. More than 120 different genotypes were detected among the 277 aphid lineages, with an average of 5.9 alleles per locus (range four to 16) and heterozygosity of 56.7%. As with previous studies of allozyme variation in aphids, most loci showed heterozygote deficits, and disequilibrium was common among allelic variants at different loci, even after removal of replicate copies of genotypes that might have been derived through clonal reproduction. Our results suggest that selection is important in structuring reproductive systems and genetic variation in French S. avenae. Canonical correspondence analysis was employed to examine the associations between genotypic and phenotypic variables, enabling the identification of alleles correlated with life-history traits.     

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).     

两性具有不同出生率和死亡率的种群动态

以往的种群动力学模型均隐含假设性比为1：1,而实际上并非总如此,不同性别的出生和死亡是不完全相同的。文中就考虑了两性具有不同出生率和死亡率的种群动态问题。可以知道种群动态只是受雌性控制,与雄性无关。     

Natural variation in social conditions affects male mate choosiness in the amphipod Gammarus roeselii

The extent of male mate choosiness is driven by a trade-off between various environmental factors associated with the costs of mate acquisition, quality assessment and opportunity costs. Our knowledge about natural variation in male mate choosiness across different populations of the same species, however, remains limited. In this study, we compared male mate choosiness across 10 natural populations of the freshwater amphipod Gammarus roeselii (Gervais 1835), a species with overall high male mating investments, and evaluated the relative influence of population density and sex ratio (both affecting mate availability) on male mate choosiness. We investigated amplexus establishment after separating mating pairs and presenting focal males with a novel, size-matched female from the same population. Our analysis revealed considerable effects of sex ratio and (to a lesser extent) population density on time until amplexus establishment (choosiness). Male amphipods are able to perceive variable social conditions (e.g., sex ratio) and modify their mating strategy accordingly: We found choosiness to be reduced in increasingly male-biased populations, whereas selectivity increases when sex ratio becomes female biased. With this, our study expands our limited knowledge on natural variations in male mate choosiness and illustrates the importance of sex ratio (i.e., level of competition) for male mating decisions in natural environments. Accounting for variation in sex ratios, therefore, allows envisioning a distinctive variation of choosiness in natural populations and highlights the importance of considering social background information in future behavioral studies.     

Genetic architecture of sexual and asexual populations of the aphid Rhopalosiphum padi based on allozyme and microsatellite markers

Cyclical parthenogens, including aphids, are attractive models for comparing the genetic outcomes of sexual and asexual reproduction, which determine their respective evolutionary advantages. In this study, we examined how reproductive mode shapes genetic structure of sexual (cyclically parthenogenetic) and asexual (obligately parthenogenetic) populations of the aphid Rhopalosiphum padi by comparing microsatellite and allozyme data sets. Allozymes showed little polymorphism, confirming earlier studies with these markers. In contrast, microsatellite loci were highly polymorphic and showed patterns very discordant from allozyme loci. In particular, microsatellites revealed strong heterozygote excess in asexual populations, whereas allozymes showed heterozygote deficits. Various hypotheses are explored that could account for the conflicting results of these two types of genetic markers. A strong differentiation between reproductive modes was found with both types of markers. Microsatellites indicated that sexual populations have high allelic polymorphism and heterozygote deficits (possibly because of population subdivision, inbreeding or selection). Little geographical differentiation was found among sexual populations confirming the large dispersal ability of this aphid. In contrast, asexual populations showed less allelic polymorphism but high heterozygosity at most loci. Two alternative hypotheses are proposed to explain this heterozygosity excess: allele sequence divergence during long-term asexuality or hybrid origin of asexual lineages. Clonal diversity of asexual lineages of R. padi was substantial suggesting that they could have frozen genetic diversity from the pool of sexual lineages. Several widespread asexual genotypes were found to persist through time, as already seen in other aphid species, a feature seemingly consistent with the general-purpose genotype hypothesis.     

Mate and territory availability affect breeding dispersal and divorce in a resident Southern House Wren Troglodytes aedon musculus population

Breeding dispersal is the movement of an individual between breeding attempts and is usually associated with the disruption of the social pair bond, although mates may disperse together as a social unit. In monogamous territorial species, the decision to disperse may be affected by individual attributes such as sex, age and condition of the disperser. However, environmental and social contexts may also play a crucial role in the decision to disperse. We analysed capture‐resighting data collected over 9 years to study breeding dispersal and divorce rates of a Southern House Wren Troglodytes aedon musculus population in South Temperate Argentina. Between‐season dispersal was more frequent than within‐season dispersal, with females dispersing more often than males, both between and within seasons. Both within‐season and between‐season breeding dispersal probability was affected by territory availability, but not by previous breeding success. When the adult sex ratio (ASR) was more skewed towards males, male between‐season dispersal was also affected by mating status, with widowed and single males dispersing more often than paired males. Within‐season divorce increased the reproductive success of females but not males, and was affected by the availability of social partners (with increasingly male‐skewed ASR). Our results suggest that territorial vacancies and mating opportunities affect dispersal and divorce rates in resident Southern House Wrens, highlighting the importance of social and environmental contexts for dispersal behaviour and the stability of social pair bonds.     

Sex ratio bias in the F1 adult progeny of African rice gall midge, Orseolia oryzivora H. and G. (Dipt., Cecidomyiidae)

Abstract:  Against the background of conflicting reports of variable sex ratio distribution in different populations of the African rice gall midge (AfRGM), Orseolia oryzivora Harris and Gagne, the number and sex ratios of F₁ adult progeny produced per female from wild and screen house populations were investigated in the screen house at 27 ± 3°C, 60–70% relative humidity and 12 : 12 h (light : dark) photoperiod. Results indicated the occurrence of a single sex-biased ratio in all the F₁ adult progeny produced per female of this pest. Regardless of the source, whether from wild or screen house population, each mated female AfRGM produced a full complement of either sex only but never both. The difference between the number of eggs laid and F₁ adult progeny that emerged per female from wild and screen house populations was not significant (P > 0.05), indicating consistency in the occurrence of this phenomenon among AfRGM populations. The reason for this could not be ascertained in this study but AfRGM eggs must be fertilized for development and eclosion to occur. No F₂ adults emerged when a cohort of emergents from the same female were allowed to re-infest susceptible seedlings, indicating absence of parthenogenesis.     

Seasonality,Population Dynamics and Production of Daphnia longispina in the Subtropical Lake Bhimtal (U.P.), India

Seasonal variation in population density, birth rate, mortality and production of a daphnid was studied in a high, altitude sub-tropical lake. A bimodal growth curve with maxima during summer and autumn was observed. A positive correlation was reported between fecundity and death rate, probably due to a difference in age specific death rate combined with a high relative frequency of juveniles in a growing population. The production of Daphnia longispina correlates better with temperature than with food. The P/B ratio was higher during summer and autumn months.     

Population genomics and geographical parthenogenesis in Japanese harvestmen (Opiliones,Sclerosomatidae, Leiobunum)

Naturally occurring population variation in reproductive mode presents an opportunity for researchers to test hypotheses regarding the evolution of sex. Asexual reproduction frequently assumes a geographical pattern, in which parthenogenesis‐dominated populations are more broadly dispersed than their sexual conspecifics. We evaluate the geographical distribution of genomic signatures associated with parthenogenesis using nuclear and mitochondrial DNA sequence data from two Japanese harvestman sister taxa, Leiobunum manubriatum and Leiobunum globosum. Asexual reproduction is putatively facultative in these species, and female‐biased localities are common in habitat margins. Past karyotypic and current cytometric work indicates L. globosum is entirely tetraploid, while L. manubriatum may be either diploid or tetraploid. We estimated species phylogeny, genetic differentiation, diversity, and mitonuclear discordance in females collected across the species range in order to identify range expansion toward marginal habitat, potential for hybrid origin, and persistence of asexual lineages. Our results point to northward expansion of a tetraploid ancestor of L. manubriatum and L. globosum, coupled with support for greater male gene flow in southern L. manubriatum localities. Specimens from localities in the Tohoku and Hokkaido regions were indistinct, particularly those of L. globosum, potentially due to little mitochondrial differentiation or haplotypic variation. Although L. manubriatum overlaps with L. globosum across its entire range, L. globosum was reconstructed as monophyletic with strong support using mtDNA, and marginal support with nuclear loci. Ultimately, we find evidence for continued sexual reproduction in both species and describe opportunities to clarify the rate and mechanism of parthenogenesis.     

Mutation and the experimental evolution of outcrossing in Caenorhabditis elegans

An understanding of the forces that contribute to the phylogenetically widespread phenomenon of sexual reproduction has posed a longstanding problem in evolutionary biology. Mutational theories contend that sex can be maintained when the deleterious mutation rate is sufficiently high, although empirical evidence is equivocal and experimental studies are rare. To test the influence of mutation on the evolution of obligate outcrossing, I introduced a genetic polymorphism for breeding system into populations of the nematode Caenorhabditis elegans with high- and low-mutation rate genetic backgrounds and tracked the change in frequency of females, hermaphrodites, and males over approximately 21 generations. Hermaphrodites invaded all populations, regardless of mutational background. However, experimental populations with elevated mutation rates experienced more outcrossing and greater retention of females. This provides experimental evidence consistent with deleterious mutational explanations for the evolution of sex in principle, but the action of other processes is required to explain the evolution of sex in entirety.     

Exploring the relationship between tychoparthenogenesis and inbreeding depression in the Desert Locust,Schistocerca gregaria

Tychoparthenogenesis, a form of asexual reproduction in which a small proportion of unfertilized eggs can hatch spontaneously, could be an intermediate evolutionary link in the transition from sexual to parthenogenetic reproduction. The lower fitness of tychoparthenogenetic offspring could be due to either developmental constraints or to inbreeding depression in more homozygous individuals. We tested the hypothesis that in populations where inbreeding depression has been purged, tychoparthenogenesis may be less costly. To assess this hypothesis, we compared the impact of inbreeding and parthenogenetic treatments on eight life‐history traits (five measuring inbreeding depression and three measuring inbreeding avoidance) in four laboratory populations of the desert locust, Schistocerca gregaria, with contrasted demographic histories. Overall, we found no clear relationship between the population history (illustrated by the levels of genetic diversity or inbreeding) and inbreeding depression, or between inbreeding depression and parthenogenetic capacity. First, there was a general lack of inbreeding depression in every population, except in two populations for two traits. This pattern could not be explained by the purging of inbreeding load in the studied populations. Second, we observed large differences between populations in their capacity to reproduce through tychoparthenogenesis. Only the oldest laboratory population successfully produced parthenogenetic offspring. However, the level of inbreeding depression did not explain the differences in parthenogenetic success between all studied populations. Differences in development constraints may arise driven by random and selective processes between populations.     

广东内伶仃岛猕猴种群年龄结构及发展趋势

根据1997年1月至2000年12月的野外观察,对内伶仃岛猕猴种群年龄结构及发展趋势进行了研究,结果表明（1）内伶钉岛猕猴种群雄性比为1：98,各年龄组性比幼年组为1：1．07,青年组为1L0．93,中壮年组为1：0．6,中老年组为1：0．57,老年组为1：1．14;（2）列出了猕猴种群年龄结构图,确定了年增长率为6．9％,种群数量的动态公式是Nt=200e^0.069t;(3)预测了未来10a的猕猴种群的数量和发展,种群数量从2001年到2010年增长到1200头左右;（4）内伶仃岛猕猴种群发展的最适容纳量为900－1100头（平均1000头）。