Isolation and characterization of self-fertile suppressors from the sterile nsdD deletion mutant of Aspergillus nidulans

To identify downstream and/or interactive factors of the nsdD gene, which encodes a positive regulator of sexual development of Aspergillus nidulans, suppressor mutants displaying a self-fertile phenotype were isolated from a sterile nsdD deletion mutant. At least five different loci (sndA-E) were identified and genetically analyzed. In the nsdD ⁺ background, most of the suppressors showed a marked increment of sexual development, even under the stress conditions that normally inhibited sexual development. The common phenotype of the suppressor mutants suggested the involvement of the snd genes in the negative regulation of sexual development in response to the environmental factors.     

The mismeasurement of sexual selection

Sexual selection can explain major micro‐ and macro‐evolutionary patterns. Much of current theory predicts that the strength of sexual selection (i) is driven by the relative abundance of males and females prepared to mate (i.e. the operational sex ratio, OSR) and (ii) can be generally estimated by calculating intra‐sexual variation in mating success (e.g. the opportunity for sexual selection, I_s). Here, we demonstrate the problematic nature of these predictions. The OSR and I_s only accurately predict sexual selection under a limited set of circumstances, and more specifically, only when mate monopolization is extremely strong. If mate monopolization is not strong, using OSR or I_s as proxies or measures of sexual selection is expected to produce spurious results that lead to the false conclusion that sexual selection is strong when it is actually weak. These findings call into question the validity of empirical conclusions based on these measures of sexual selection.     

Relative growth before and after sexual maturity in Ocypode platytarsis (Milne Edwards) and Ocypode cordimana (Desmarest) (Crustacea: Decapoda)

The relative growth of two species of the genus Ocypode inhabiting different ecological niches was studied before and after sexual maturity. The growth coefficient of both sexes of O. cordimana, which inhabits supra-littoral zones, is higher than that of O. platytarsis found in wave-wash zones.s

Some characteristics such as eyestalk length, major chela width and obdomen width do not show any difference in the growth pattern of either species of Ocypode at the onset of sexual maturity. Other characteristics such as minor and major chela length in male O. platytarsis; carapace length, minor chela length and width in female O. platytarsis; major chela length and third walking leg length in male O. cordimana showed a difference in the growth pattern after the onset of sexual maturity.

Major chela length of males of both species is clearly a sexually dimorphic feature since it showed an increase in the growth coefficient after sexual maturity. Merus length in male O. platytarsis and minor chela length in female O. cordimana also showed such sexual dimorphism.

There are several characteristics in which the growth coefficient declines after sexual maturity. The change in growth coefficient of different characteristics may depend on the adaptive value of the characteristic in reproductive or post-reproductive activities of O. cordimana and O. platytarsis.     

THE GENETIC BASIS OF SEXUAL ISOLATION BETWEEN DROSOPHILA MELANOGASTER AND D. SIMULANS

The genetic analysis of sexual isolation between the closely-related species Drosophila melanogaster and Drosophila simulans involved two experiments with no-choice tests. The efficiency of sexual isolation was measured by the frequency of courtship initiation and interspecific mating. We first surveyed the variation in sexual isolation between D. melanogaster strains and D. simulans strains of different geographic origin. Then, to investigate variation in sexual isolation within strains, we made F₁ diallel sets of reciprocal crosses within strains of D. melanogaster and D. simulans. The F₁ diallel progeny of one sex were paired with the opposite sex of the other species. The first experiment showed significant differences in the frequency of interspecific mating between geographic strains. There were more matings between D. simulans females and D. melanogaster males than between D. melanogaster females and D. simulans males. The second experiment uncovered that the male genotypes in the D. melanogaster diallel significantly differed in interspecific mating frequency, but not in courtship initiation frequency. The female genotypes in the D. simulans diallel were not significantly different in courtship initiation and interspecific mating frequency. Genetic analysis reveals that in D. melanogaster males sexual isolation was not affected by either maternal cytoplasmic effects, sex-linked effects, or epistatic interaction. The main genetic components were directional dominance and overdominance. The F₁ males achieved more matings with D. simulans females than the inbred males. The genetic architecture of sexual isolation in D. melanogaster males argues for a history of weak or no selection for lower interspecific mating propensity. The behavioral causes of variation in sexual isolation between the two species are discussed.     

Sexual dimorphism in the skull geometry of newt species of <Emphasis Type="Italic">Ichthyosaura,Triturus</Emphasis> and <Emphasis Type="Italic">Lissotriton</Emphasis> (Salamandridae,Caudata, Amphibia)

In this study, we applied geometric morphometrics to explore variations in the level and pattern of sexual size dimorphism (SSD) and sexual shape dimorphism (SShD) of the ventral cranium in three different Modern Eurasian newt taxa (Ichthyosaura alpestris, Triturus species group and Lissotriton vulgaris). The ventral cranium is the part of the skull that is more directly related to foraging and feeding. Our results indicate that the level and pattern of sexual dimorphism in the ventral cranium differ among Modern Eurasian newt taxa. Regarding sexual dimorphism in skull size, Ichthyosaura alpestris and Triturus species show female-biased patterns (females are larger than males), whereas Lissotriton vulgaris appears to be non-dimorphic in skull size. In I. alpestris and Triturus species, SShD is mostly absent, whereas in L. vulgaris, SShD is more pronounced. A high level of variation between populations in both SSD and SShD indicates that local conditions may have a profound effect on the magnitude and direction of sexual dimorphism. The significant sexual differences in ventral cranium size and shape indicate possible subtle intersexual differences in ecological demands due to diet specialisation, in spite of similar general ecological settings.     

Reproductive and sexual characteristics in five Lethrinus species in waters off the Ryukyu Islands

Gonads of five lethrinids, viz., Lethrinus harak, L. miniatus, L. obsoletus, L. ornatus, and L. sp. 2, were collected monthly in waters off the Ryukyu Islands and observed histologically to reveal their spawning periods and size at sexual maturation and sexual transition. The spawning period was from April to November for L. harak, from April to July for L. miniatus, from April to October for L. obsoletus, from May to November for L. ornatus, and from April to October for L. sp. 2. Sexual patterns in the five species were determined by fork length at both sexual maturation and sexual transition. The body size (fork length, FL) and sex ratio (% of female) at 90% maturity in L. harak were 21.1 cm FL and 90%, respectively; those of L. miniatus were 42.2 cm FL and 80%; 25.7 cm FL and 60% for L. obsoletus; about 20 cm FL and 90% for L. ornatus; and about 26 cm FL and 90% for L. sp. 2. Because the sex ratios decreased to 0% at the maximum size classes in L. miniatus, L. ornatus, and L. sp. 2, the sexual patterns in these species were considered to be protogynous hermaphrodite. Although the sex ratio (% of female) once decreased to about 30% at 28 cm FL, rapid increase occurred in the larger size class in L. harak. However, the increase did not result from sexual transition. Hence, the sexual pattern of L. harak was considered to be protogynous hermaphrodite. Because the sex ratio (% of female) at body sizes larger than 23 cm FL was stable at about 60% in L. obsoletus, the sexual pattern was determined to be one of juvenile hermaphrodites.     

INTER-THALLIC SEXUAL REACTIONS IN ACHLYA,A GENUS OF THE AQUATIC FUNGI

Barksdale , Alma Whiffen . (New York Botanical Garden, New York.) Inter-thallic sexual reactions in Achlya, a genus of the aquatic fungi . Amer. Jour. Bot. 47(1): 14—23. Illus. 1960.– Inter-thallic sexual reactions, involving the coöperative induction of sexual organs, have been found to occur between members of homothallic, as well as heterothallic species of the sub-genus Achlya. The induction of sexual organs, both on single thalli and on paired thalli, is temperature-dependent. Sexual organs were initiated at 15°—20°C. in 14 of 27 heterothallic strains that were usually sterile at 25°. The hermaphroditic sexual organs were like those of either A. ambisexualis or A. bisexualis in 19 strains; like those of A. prolifera in 3 strains; like those of A. inflata in 1 strain; and like those of A. americana in 1 strain. In some instances, the sexual reaction between paired thalli was complete at 20°C. but incomplete (i.e., oospores were not formed) at 25°C. The heterothallic and homothallic strains studied could be assigned to 1 of 12 (6 female and 6 male) inter-action types, on the basis of their sexual responses. These sexual types are characterized and their possible usefulness in indicating kinship among species of Achlya is assessed. The possible origin of heterothallic from homothallic forms of Achlya is discussed.     

Different Responses of Sexual and Asexual Reproduction of Arundinella hirta to Flooding

The sexual and asexual reproductive features of the graminoid species Arundinella hirta growing at riversides of the Jialing River were analyzed. It was found that the total seed mass, seed number per plant, and sexual reproductive allocation of A. hirta decreased with decreasing bank elevation, and plants growing at the lowest elevations of banks subjected to intense flooding did not show sexual reproduction. The total plant biomass and the number of ramifications per plant increased with flooding intensity, which implies that, contrary to sexual reproduction, asexual reproduction of A. hirtawas enhanced by flooding. Translated from the Journal of Southwest China Normal University (Natural Science), 2005, 30(2) (in Chinese)     

EVOLUTION OF ASEXUALITY VIA DIFFERENT MECHANISMS IN GRASS THRIPS (THYSANOPTERA: Aptinothrips)

Asexual lineages can derive from sexual ancestors via different mechanisms and at variable rates, which affects the diversity of the asexual population and thereby its ecological success. We investigated the variation and evolution of reproductive systems in Aptinothrips, a genus of grass thrips comprising four species. Extensive population surveys and breeding experiments indicated sexual reproduction in A. elegans, asexuality in A. stylifer and A. karnyi, and both sexual and asexual lineages in A. rufus. Asexuality in A. stylifer and A. rufus coincides with a worldwide distribution, with sexual A. rufus lineages confined to a limited area. Inference of molecular phylogenies and antibiotic treatment revealed different causes of asexuality in different species. Asexuality in A. stylifer and A. karnyi has most likely genetic causes, while it is induced by endosymbionts in A. rufus. Endosymbiont‐community characterization revealed presence of Wolbachia, and lack of other bacteria known to manipulate host reproduction. However, only 69% asexual A. rufus females are Wolbachia‐infected, indicating that either an undescribed endosymbiont causes asexuality in this species or that Wolbachia was lost in several lineages that remained asexual. These results open new perspectives for studies on the maintenance of mixed sexual and asexual reproduction in natural populations.     

OPERATIONAL SEX RATIO BUT NOT DENSITY AFFECTS SEXUAL SELECTION IN A FISH

The operational sex ratio (OSR) and density are considered important factors affecting the strength of sexual selection. Although there is increasing evidence that OSR and density affect the potential for sexual selection, few studies have addressed whether this is realized in phenotypic selection and how the two factors interact. We manipulated OSR (three levels) and male density (two levels) in 36 experimental breeding populations of Gobiusculus flavescens—a fish with paternal care. We measured mating competition behavior, the opportunity for selection (I), and selection on four morphological traits in males. We found sexual selection on two male traits, with the strongest selection being 20% of I. As predicted from OSR theory, increasing female scarcity caused males to become more competitive, concomitant with an increase in I and selection on morphological traits. Model simulations of I based on random mating (I_min) and maximum mate monopolization (I_max) demonstrated that the potential for sexual selection was close to its theoretical maximum across the range of OSRs. However, male density and its interaction with the OSR did not affect sexual selection. We argue that a multifaceted approach, combining mating behavior and selection analyses, can help us to understand how ecological factors affect sexual selection.     

A review of chromosomal variation in Dugesia japonica and D. ryukyuensis in the Far East

The chromosome numbers of Dugesia japonica Ichikawa et Kawakatsu, 1964, are n = 8, 2x = 16 and 3x = 24; those of Dugesia ryukyuensis Kawakatsu, 1976, are n = 7, 2x = 14 and 3x = 21. The karyotypes of both species include diploid, triploid and mixoploid; aneuploidic and mixoaneuploidic karyotypes may occur. In 785 specimens studied of D. japonica, the occurrence rates of specimens having each karyotype are substantially the same (29–37%). Diploid sexual specimens represented nearly 10% of the total and virtually no triploid or mixoploid sexual specimens were found. The diploid karyotype can be inherited by both sexual and asexual reproduction; the triploid and mixoploid karyotypes will be inherited only by asexual reproduction. In 51 specimens studied of D. ryukyuensis, the different karyotypes are diploid (ca 39%), triploid (ca 57%) and mixoploid (ca 4%). Diploid sexual specimens represented nearly 25% of the total; sexual specimens with tripooidic karyotypes made up nearly 27%. The diploid, triploid and mixoploid karyotypes were also found in juveniles hatched from cocoons. The diploid karytyype is inherited by both sexual and asexual reproductions; the other karyotypes may be inherited by parthenogenesis or self-fertilization (including pseudogamy) and asexual reproduction.     

Clonal diversity and polyphyletic origins of hybrid and spontaneous parthenogenetic Campeloma (Gastropoda: Viviparidae) from the south-eastern United States

Some theories for the maintenance of sexual reproduction indicate that parthenogens may persist if there is high clonal diversity and high dispersal rates. Using allozymic variation, we report on the origin, clonal diversity and population structure of hybrid and spontaneous parthenogens from south-eastern United States populations of the freshwater snail Campeloma. Independent origins of triploid hybrid parthenogens in the Florida panhandle occurred by hybridization between an Atlantic coastal species (C. limum or C. floridense) and the Florida sexual species (C. geniculum). Allozyme genotypic diversity is similar between these hybrid parthenogens and sexuals. Diploid spontaneous parthenogens originated multiple times from nonlocal C.limum sexual populations in Atlantic coastal rivers, and levels of genotypic diversity are significantly higher in sexual C. limum. How parthenogens originate, the degree of clonal diversity, and their subsequent dispersal influence whether basic assumptions of evolution-of-sex models are met.     

Comparing measures of breeding inequality and opportunity for selection with sexual selection on a quantitative character in bighorn rams

The reliability and consistency of the many measures proposed to quantify sexual selection have been questioned for decades. Realized selection on quantitative characters measured by the selection differential i was approximated by metrics based on variance in breeding success, using either the opportunity for sexual selection I_s or indices of inequality. There is no consensus about which metric best approximates realized selection on sexual characters. Recently, the opportunity for selection on character mean OSM was proposed to quantify the maximum potential selection on characters. Using 21 years of data on bighorn sheep (Ovis canadensis), we investigated the correlations between seven indices of inequality, I_s, OSM and i on horn length of males. Bighorn sheep are ideal for this comparison because they are highly polygynous and sexually dimorphic, ram horn length is under strong sexual selection, and we have detailed knowledge of individual breeding success. Different metrics provided conflicting information, potentially leading to spurious conclusions about selection patterns. I_δ, an index of breeding inequality, and, to a lesser extent, I_s showed the highest correlation with i on horn length, suggesting that these indices document breeding inequality in a selection context. OSM on horn length was strongly correlated with i, I_s and indices of inequality. By integrating information on both realized sexual selection and breeding inequality, OSM appeared to be the best proxy of sexual selection and may be best suited to explore its ecological bases.     

Seasonal changes in cuticular hydrocarbons in response to polyphenism in the host-alternating aphid Prociphilus oriens

In most terrestrial arthropods, cuticular hydrocarbons (CHCs) function to assist in desiccation tolerance and chemical communications. However, few studies have clarified whether CHC profiles change among developmental stages or among different morphs in non-social insects. In the present study, we evaluated how CHC profiles change in accordance with polyphenism in the host-alternating aphid Prociphilus oriens, which exhibits a complex life cycle and five distinct morphs. These morphs are sexual or asexual and adapt to different host plants. We found that all generations of P. oriens shared high proportions of n-alkanes, but its composition varied among morphs. Three morphs that are attended by ants were characterized by relatively high proportions of n-C25 to n-C27, whereas two morphs that are not attended by ants had higher proportions of longer-chain n-alkanes, such as n-C27 and n-C29. The CHC profiles of sexual females were largely different from those of males. Considering that sexual females of Prociphilus spp. lack organs that secrete sex pheromones (scent plaques), the CHCs of sexual females are likely to function as a sex attractant. High proportions of methyl-branched alkanes were detected in the long and flocculent waxy substances of autumnal migrants. These methyl-branched alkanes are considered a cue to recognize conspecifics. We concluded that the functions and components of CHCs differ among morphs, and that those of sexual females differ from those of males and asexual generations because of their function in sexual communication.     

Sexual selection has minimal impact on effective population sizes in species with high rates of random offspring mortality: An empirical demonstration using fitness distributions

The effective population size (N_e) is a fundamental parameter in population genetics that influences the rate of loss of genetic diversity. Sexual selection has the potential to reduce N_e by causing the sex‐specific distributions of individuals that successfully reproduce to diverge. To empirically estimate the effect of sexual selection on N_e, we obtained fitness distributions for males and females from an outbred, laboratory‐adapted population of Drosophila melanogaster. We observed strong sexual selection in this population (the variance in male reproductive success was ～14 times higher than that for females), but found that sexual selection had only a modest effect on N_e, which was 75% of the census size. This occurs because the substantial random offspring mortality in this population diminishes the effects of sexual selection on N_e, a result that necessarily applies to other high fecundity species. The inclusion of this random offspring mortality creates a scaling effect that reduces the variance/mean ratios for male and female reproductive success and causes them to converge. Our results demonstrate that measuring reproductive success without considering offspring mortality can underestimate N_e and overestimate the genetic consequences of sexual selection. Similarly, comparing genetic diversity among different genomic components may fail to detect strong sexual selection.     

Correlated paternity measures mate monopolization and scales with the magnitude of sexual selection

Indirect measures of sexual selection have been criticized because they can overestimate the magnitude of selection. In particular, they do not account for the degree to which mating opportunities can be monopolized by individuals of the sex that compete for mates. We introduce a measure of mate monopolization (m) based on the magnitude of correlated paternity and evaluate its ability to track changes in the magnitude of sexual selection. Simulation models were used to compare how well m tracked changes in the selection differential (s) for a trait regulating mating success. We further evaluated the association between m and s using ten replicate mating arrays of Sagittaria latifolia in which plants with contrasting alleles at microsatellite loci competed for siring opportunities. The computer models and mating arrays both demonstrated a positive linear association between m and s, supporting the utility of m as an index of sexual selection. Commonly used measures of sexual selection are not easily applied to organisms, such as the flowering plants, for which mating events are difficult to observe. The measure of mate monopolization introduced here could prove to be a useful addition to studies of sexual selection in these organisms.     

Sexual dimorphism and interspecific cranial form in two capuchin species: Cebus albifrons and C. apella

Ontogenetic patterns of sexual dimorphism and cranial form in two capuchin monkeys, Cebus albifrons and C. apella, are investigated by means of univariate, bivariate, and multivariate statistics. The analyses are based on 23 linear variables. Univariate analyses indicate that similar ontogenetic patterns of cranial sexual dimorphism are present; however, interspecific differences exist in timing. Ontogenetic scaling is present in both species' crania; however, it is more prevalent in C. albifrons. Several departures are present in cranial regions associated with orbital shape, the dental arcade, and the muscles of mastication. The latter two indicate that sexual differences in diet and/or foraging strategies may exist. Sexual selection is suggested as being the primary selective regime underlying the observed patterns of cranial sexual dimorphism in each species. Interspecific comparisons confirm that C. apella possesses a more dimorphic cranium than C. albifrons and that sexual dimorphism in C. apella begins earlier in development. Although interspecific ontogenetic scaling is present in some cranial variables, C. apella is not just a scaled-up version of C. albifrons. These sympatric congeners seem to be differentiated by variables related to the orbital region and the masticatory apparatus, as indicated by both departures from ontogenetic scaling and results of the discriminant function analysis. Ecological selection, rather than varying degrees of sexual selection, is likely to be responsible for this finding given that C. apella is known to consume hard-object foods. This is consistent with the predicted outcome of the competitive exclusion principle. Am J Phys Anthropol 104:487–511, 1997. © 1997 Wiley-Liss, Inc.     

A nonlethal method of identification of Ambystoma laterale,A. jeffersonianum and sympatric unisexuals

Unisexual salamanders in the genus Ambystoma live in obligate sympatry with the sexual species Ambystoma laterale or Ambystoma jeffersonianum, from which they are not easily distinguished. Because the sexual species are protected in many parts of their range, accurate identification of sexual and unisexual individuals is required for conservation purposes. Unisexual individuals are currently identified using isozyme electrophoresis, which requires sacrificing the animal. Here we present a nonlethal method of identification for members of the A. laterale–jeffersonianum part of the Ambystoma complex utilizing genome‐specific microsatellites with or without the addition of flow cytometry.     

Germline stem cells are critical for sexual fate decision of germ cells

Egg or sperm? The mechanism of sexual fate decision in germ cells has been a long‐standing issue in biology. A recent analysis identified foxl3 as a gene that determines the sexual fate decision of germ cells in the teleost fish, medaka. foxl3/Foxl3 acts in female germline stem cells to repress commitment into male fate (spermatogenesis), indicating that the presence of mitotic germ cells in the female is critical for continuous sexual fate decision of germ cells in medaka gonads. Interestingly, foxl3 is found in most vertebrate genomes except for mammals. This provides the interesting possibility that the sexual fate of germ cells in mammals is determined in a different way compared to foxl3‐possessing vertebrates. Considering the fact that germline stem cells are the cells where foxl3 begins to express and sexual fate decision initiates and mammalian ovary does not have typical germline stem cells, the mechanism in mammals may have been co‐evolved with germline stem cell loss in mammalian ovary.