Detecting the undetected: estimating the total number of loci underlying a quantitative trait

Recent studies have begun to reveal the genes underlying quantitative trait differences between closely related populations. Not all quantitative trait loci (QTL) are, however, equally likely to be detected. QTL studies involve a limited number of crosses, individuals, and genetic markers and, as a result, often have little power to detect genetic factors of small to moderate effects. In this article, we develop an estimator for the total number of fixed genetic differences between two parental lines. Like the Castle-Wright estimator, which is based on the observed segregation variance in classical crossbreeding experiments, our QTL-based estimator requires that a distribution be specified for the expected effect sizes of the underlying loci. We use this expected distribution and the observed mean and minimum effect size of the detected QTL in a likelihood model to estimate the total number of loci underlying the trait difference. We then test the QTL-based estimator and the Castle-Wright estimator in Monte Carlo simulations. When the assumptions of the simulations match those of the model, both estimators perform well on average. The 95% confidence limits of the Castle-Wright estimator, however, often excluded the true number of underlying loci, while the confidence limits for the QTL-based estimator typically included the true value approximately 95% of the time. Furthermore, we found that the QTL-based estimator was less sensitive to dominance and to allelic effects of opposite sign than the Castle-Wright estimator. We therefore suggest that the QTL-based estimator be used to assess how many loci may have been missed in QTL studies.     

The genetic basis of interspecific differences in genital morphology of closely related carabid beetles

Marked diversification of genital morphology is common in internally fertilizing animals. Although sexual selection may be the primary process controlling genital evolution, factors promoting genital evolution are controversial, and the genetic background of genital morphology is poorly understood. We analyzed the genetic basis of species-specific genital morphologies in carabid beetles of the subgenus Ohomopterus (genus Carabus, Carabidae) using two parapatric species with hybrid zones. Biometric analyses on experimental F(1) and backcross populations revealed that inheritance of genital morphology is polygenic. Applying Lande's modification of the Castle-Wright estimator to population means and variances to estimate the minimum number of genes involved, we found that a relatively small number of loci is responsible for species differences in genital morphology. In addition, joint-scaling tests indicated that the additive genetic effect accounts for most interspecific differences in genital traits, but dominance and epistatic genetic effects also play roles. Overall, the genetic basis of male and female genitalia is fairly simple, enabling these traits to respond quickly to selection pressures and to diverge rapidly. Our results provide insight into the diversification of genital morphology in carabid beetles, and will hopefully stimulate further studies on the genetic basis of genitalia, such as mapping of quantitative trait loci affecting species-specific genital morphology.     

Estimating numbers of EMS-induced mutations affecting life history traits in Caenorhabditis elegans in crosses between inbred sublines

Inbred lines of the nematode Caenorhabditis elegans containing independent EMS-induced mutations were crossed to the ancestral wild-type strain (N2). Replicated inbred sublines were generated from the F1 offspring under conditions of minimal selection and, along with the N2 and mutant progenitor lines, were assayed for several fitness correlates including relative fitness (w). A modification of the Castle-Wright estimator and a maximum-likelihood (ML) method were used to estimate the numbers and effects of detectable mutations affecting these characters. The ML method allows for variation in mutational effects by fitting either one or two classes of mutational effect, and uses a Box-Cox power transformation of residual values to account for a skewed distribution of residuals. Both the Castle-Wright and the ML analyses suggest that most of the variation among sublines was due to a few (approximately 1.5-2.5 on average) large-effect mutations. Under ML, a model with two classes of mutational effects, including a class with small effects, fitted better than a single mutation class model, although not significantly better. Nonetheless, given that we expect there to be many mutations induced per line, our results support the hypothesis that mutations vary widely in their effects.     

Female polyandry affects their sons’ reproductive success in the red flour beetle Tribolium castaneum

A potential benefit to females mating with multiple males is the increased probability that their sons will inherit traits enhancing their pre‐ or post‐mating ability to obtain fertilizations. We allowed red flour beetle (Tribolium castaneum) females to mate on three consecutive days either repeatedly to the same male or to three different males. This procedure was carried out in 20 replicate lines, 10 established with wild‐type, and 10 with the Chicago black morph, a partially dominant phenotypic marker. The paternity achieved by the sons of females from polyandrous vs. monandrous lines of contrasting morph was assessed in the F1, F2 and F3 generation by mating wild‐type stock females to two experimental males and assigning the progeny to either sire based on phenotype. The sons of polyandrous wild‐type females achieved significantly higher paternity when mating in the second male role than the sons of monandrous wild‐type females. By contrast, when mating in the first male role, males produced by females from polyandrous lines tended to have lower paternity than males from monandrous lines. Both effects were independent of the number of mates of the black competitor’s mother, and interacted significantly with the number of progeny laid by the female. These results provide the first evidence that manipulating the number of mates of a female can influence her sons’ mating success and suggest a potential trade‐off between offence and defence in this species.     

Molecular approaches identify known species, reveal cryptic species and verify host specificity of Chinese Philotrypesis (Hymenoptera: Pteromalidae)

Philotrypesis, a major component of the fig wasp community (Hymenoptera: Pteromalidae), is a model taxon for studying male fighting and mating behaviour. Its extreme sexual dimorphism and male polymorphism render species identification uncertain and in-depth research on its ecology, behaviour and other evolutionary topics challenging. The fig wasps' enclosed habitat within the syconia makes their mating behaviour inaccessible, to the extent of matching conspecific females and males. In this study, we combine morphological and molecular analyses to identify species of Philotrypesis sampled from south China and to associate their extraordinarily dimorphic genders and labile male morphologies. Morphological evaluations of females identify 22 species and 28 male morphs. The mitochondrial cytochrome c oxidase I and nuclear internal transcribed spacer 2 data detect 21 species using females, and 15 species among the males. Most of the males match the species as delimited by females. Both markers reveal cryptic species in P. quadrisetosa on Ficus vasculosa. Most species of wasps live on one species of fig but three species co-occur in two hosts (F. microcarpa and F. benjamina), which indicates host switching.     

Why some male Mongolian gerbils may help at the nest: testosterone, asexuality and alloparenting

In previous studies we have shown that those male Mongolian gerbils, Meriones unguiculatus, that as fetuses resided in intrauterine positions (IUPs) located between two female fetuses (2F males) have lower circulating levels of testosterone, less well-developed genital musculature, and lower reproductive success than males gestated in IUPs between two male fetuses (2M males). We have also found that such 2F males spend more time caring for conspecific young than do 2M males, and that presence of a 2F male, but not of a 2M male, with a lactating female and her litter decreases the cost to a dam of suckling one litter while gestating a second litter conceived in postpartum oestrus. Here we show that some 2F males, those with circulating levels of testosterone similar to those seen in females of their species, show no interest in females in oestrus and fail to impregnate females with which they are paired. Such 'asexual' 2F males spend 30-50% more time caring for nestlings than do sexually active 2F males. We suggest that such asexual, highly parental 2F males are incapable of direct reproduction and are obligate helpers at the nest that can contribute to their own fitness only by assisting to rear collateral kin. Copyright 2000 The Association for the Study of Animal Behaviour.     

A small number of genes underlie male pigmentation traits in Lake Malawi cichlid fishes

Pigmentation patterns are one of the most recognizable forms of phenotypic diversity and an important component of organismal fitness. While much progress has been made in understanding the genes controlling pigmentation in model systems, many questions remain about the genetic basis of pigment traits observed in nature. Lake Malawi cichlid fishes are known for their diversity of male pigmentation patterns, which have been shaped by sexual selection. To begin the process of identifying the genes underlying this diversity, we quantified the number of pigment cells on the body and fins of two species of the genus Metriaclima and their hybrids. We then used the Castle-Wright equation to estimate that differences in individual pigmentation traits between these species are controlled by one to four genes each. Different pigmentation traits are highly correlated in the F(2) , suggesting shared developmental pathways and genetic pleiotropy. Melanophore and xanthophore traits fall on opposite ends of the first principal component axis of the F(2) phenotypes, suggesting a tradeoff during the development of these two pigment cell types.     

Evidence of doubly uniparental inheritance of the mitochondrial DNA in Polititapes rhomboides (Bivalvia,Veneridae): Evolutionary and population genetic analysis of F and M mitotypes

Doubly uniparental inheritance (DUI) is a particular mitochondrial DNA inheritance mode reported in a number of bivalves. DUI species show two types of mtDNA, one transmitted from females to daughters and sons (F mitotype) and another one from males to sons (M mitotype). In Veneridae, the existence of DUI has been investigated in several species but it was found in only two of them. In this study, we obtained partial sequences of rrnL, cytb and cox1 genes of males and females of Polititapes rhomboides from NW Spain and we demonstrated the existence of heteroplasmy in males, as expected under DUI. F and M mitotypes showed a taxon-specific phylogenetic pattern and similar evolutionary rates. We focused on cox1 for population genetic analysis, examining separately F and M mitotypes, but also F mitotypes from females (F_♀) and males (F_♂). In all cases, cox1 bears signs of strong purifying selection, with no apparent evidence of relaxed selection in the M genome, while the divergence between F and M genomes is in agreement with the neutral model of evolution. The cox1 polymorphism, higher at the M than at the F genome, also shows clear footprints of genetic hitchhiking with favourable mutations at other mtDNA loci, except for F_♂. In terms of population structure, results suggest that the pattern depends on the examined mitotype (F, F_♀, F_♂ or M).     

Structure and function of the Johnston’s organ in <Emphasis Type="Italic">Fleuria lacustris</Emphasis> Kieff. males (Diptera,Chironomidae)

The majority of chironomid midges mate in swarms, where males find females by their flight sounds using specialized Johnston’s organs for acoustic perception. Males of Fleuria lacustris do not swarm and copulate on the ground. Both the flagellum and antennal fibrils are shortened and the pedicel is reduced in comparison with that typical of swarming midge species. Our results demonstrate that, in swarming midge species, the number of antennal fibrils and of A-type chordotonal sensilla in the male Johnston’s organ is by 1.33 and by 21 times higher, respectively, than in F. lacustris. Though Johnston’s organ of the latter species responds to female flight tones, it is significantly (about 70 times) less sensitive to these stimuli in comparison with the Johnston’s organ of swarming species. The decrease in the sensitivity of the Johnston’s organ in F. lacustris can be explained by the decrease in the size and the number of antennal structures, whereas their ability to perceive acoustic signals is determined by the presence of a relatively high number of A-type sensilla. Our results demonstrate that F. lacustris males are not able to search for conspecific females by perception of acoustic signals produced during the flight; however, it cannot be ruled out that females produce sounds during copulation on the substrate and thus affect male behavior.     

Hormonal control of birth behavior in the Tasmanian devil Sarcophilus harrisii

In a number of marsupial species, females exhibit characteristic, stereotyped parturient behavior that facilitates the passage of the neonates to the pouch. In macropodids, this parturient behavior can be induced in non-pregnant females and males by treatment with either prostaglandin F2alpha (PGF2alpha) or oxytocin (OT). This study investigated the effects of PGF2alpha and OT on behavior of Tasmanian devils. Animals tended to sit or lie down quickly, with little vocalization, after treatment with PGF2alpha or OT, while after saline, the animals remained alert, seldom sat, and frequently vocalized. Hormone treatment caused increased respiration. Urogenital and pouch grooming, a characteristic element of parturient behavior in macropodids, was seen in only one devil after hormone treatment. However, no pouch or urogenital grooming was seen in videotape of a devil giving birth, so this may not be a feature of parturient behavior in this species. Overall behavior of males and females was very similar suggesting that the behavioral effects observed may be due to direct neural action of PGF2alpha or OT, rather than an indirect response to uterine or vaginal contractions caused by the hormones. This study is the first to demonstrate that OT results in PGF2alpha secretion as PGFM levels rose after OT injection.     

Sex determination and the evolution of dioecy from monoecy in Sagittaria latifolia (Alismataceae)

The role of mutations of small versus large effect in adaptive evolution is of considerable interest to evolutionary biologists. The major evolutionary pathways for the origin of dioecy in plants (the gynodioecy and monoecy-paradioecy pathways) are often distinguished by the number of mutations involved and the magnitude of their effects. Here, we investigate the genetic and environmental determinants of sex in Sagittaria latifolia, a species with both monoecious and dioecious populations, and evaluate evidence for the evolution of dioecy via gynodioecy or monoecy-paradioecy. We crossed plants of the two sexual systems to generate F1, F2 and backcross progeny, and grew clones from dioecious populations in low-and high-fertilizer conditions to examine sex inconstancy in females and males. Several lines of evidence implicate two-locus control of the sex phenotypes. In dioecious populations sex is determined by Mendelian segregation of alleles, with males heterozygous at both the male- and female-sterility loci. In monoecious populations, plants are homozygous for alleles dominant to male sterility in females and recessive to female sterility in males. Experimental manipulation of resources revealed sex inconstancy in males but not females. These results are consistent with predictions for the evolution of dioecy via gynodioecy, rather than the expected monoecy-paradioecy pathway, given the ancestral monoecious condition.     

Aspects related to productivity for four generations of a Lutzomyia longipalpis laboratory colony

A closed colony of Lutzomyia longipalpis was established with specimens collected in the Raposa - Serra do Sol indian reservoir, one of the main foci of visceral leishmaniasis in the State of Roraima, Brazil. Biological observations were made on four generations of a L. longipalpis colony with emphasis on productivity. Aspects studied were the number of laid and retained eggs, and the number of adults (male and female) per generation. During the four generations the percentage of engorged females that laid eggs varied from 64.2% (third generation-F3) to 90.3% (second generation-F2). The mean number of eggs laid per female varied from 23.6 (F3) to 39. 9 (first generation-F1). The maximum number of eggs laid per female varied from 84 (F3) to 124 (F1). The mean number of retained eggs per female was 12.7 (parental generation-P and F1) to 22.1 (F2). The number of females exceeded the number of males in all generations. However, significant difference for male/female ratio was found only for F3. Fecundity rates were between 42.1 (F3) and 58.3 (F2). From a total of 439 blood-fed females, 355 females laid 12,257 eggs that yield 5,354 adults (2,525 males and 2,829 females) in four generations. F2 presented maximum productivity and fecundity rates.     

Social organization of white‐headed langurs (Trachypithecus leucocephalus) in the Nongguan Karst Hills,Guangxi, China

The number of males per group is the most variable aspect of primate social organization and is often related to the monopolizability of females, which is mainly determined by the number of females per group and their reproductive synchrony. Colobines show both inter‐specific and intra‐specific variations in the number of males per group. Compared with other colobine species, little is known about the social organization of white‐headed langur (Trachypithecus leucocephalus), despite its endangered status and unusual limestone habitat. As a part of a long‐term study of the white‐headed langurs in the Nongguan Karst Hills, Guangxi, China, we quantitatively investigated their social organization by analyzing census data from 1998 to 2003. The population censuses revealed that the predominant social organization of bisexual groups was the one‐male group, similar to a previous report on this species and many other Asian colobines. In such groups, one adult male associated with 5.1 adult females, 0.1 sub‐adult males, 2.6 juveniles and 2.9 infants on average, with a mean group size of 11.7 individuals. In addition, three multi‐male groups were recorded, consisting of 2–3 adult males, 1–5 adult females, 0–2 sub‐adult males, 0–7 juveniles and 0–2 infants. They did not contain more adult females than the one‐male groups and were unstable in group membership. The langurs outside bisexual groups were organized into small nonreproductive groups or lived as solitaries. The nonreproductive groups averaged 1.3 adult males, 1.3 sub‐adult males and 2.6 juveniles. Juvenile females were present in such groups on 52.4% of all occasions. As predicted by the monopolization model, the prevalence of the one‐male pattern in this species may mainly be attributed to the small number of females in the group. The possible reasons for the occurrence of multi‐male groups and the presence of juvenile females in nonreproductive groups are also discussed. Am. J. Primatol. 71:206–213, 2009. © 2008 Wiley‐Liss, Inc.     

Divergent abdominal bristle patterns in two distantly related drosophilids: antero-posterior variations and sexual dimorphism in a modular trait

The number of neurosensory bristles on abdominal sternites of Drosophila is a most investigated trait for quantitative genetic studies. However, the developmental pattern expressed on successive segments in both sexes has remained so far a neglected field. We explored three aspects of this general problem with an isofemale line design: comparing two distantly related species, Drosophila melanogaster and Zaprionus indianus, investigating bristle number variation along the antero-posterior axis, and analysing the sexual dimorphism. Antero-posterior variations could be analysed from segment A2 to A7 in females, and A2-A5 in males. In D. melanogaster, males and females showed parallel changes with a consistently lower number in males. In Z. indianus females the number was quite stable along the abdomen, while in males an important antero-posterior increase was found. The sexual dimorphism was further analysed by considering the female-male correlation and the female/male ratio. The results suggest that sternite bristle number is determined by several developmental genetic systems. One is acting along the antero-posterior axis and may be associated to a gradient, since the genetic correlation decreases when more distant segments are compared. Another is acting in the same way on most segments of both sexes, since the female-male genetic correlation is similar between homologous and non-homologous segments. Finally, genes with specific sex effects are acting on A7 in females of both species, and on A5 in Z. indianus males. The overall architecture of female and male abdomen seems to be constrained by the development of reproductive organs. A large difference between species suggests, however, that the sexual dimorphism of abdominal bristle number is not evolutionarily constrained.     

雌性斑胸草雀发育和成体期发声核团LMAN中神经元数量的高水平维持

鸟类的发声和发声学习涉及处于脑中不同水平的相互联系的一些发声核团。在很多鸟类中, 仅雄性鸣啭, 例如在斑胸草雀(Taeniogygia guttata) 中, 雌雄个体间存在显著的性差异, 雌性发声系统和发声核团中的神经元数量显著少于雄性。推测这种性差异也可能存在于弓状皮质前部大细胞外侧核(LMAN), 但有关这方面的报道还很少。为探讨雌雄鸣禽脑中神经元数量发育的规律, 我们应用半薄切片对斑胸草雀发育不同阶段的神经元数量和密度的变化进行了数量分析。结果表明, 在全部实验组中, 发育10 d和成体雌性LMAN中神经元的密度和神经元数量均无明显变化, 神经元的数量维持在一个较高水平。在雄鸟中, 我们发现在发育早期神经元的数量有极为显著的减少, 这与已有的报道, 即应用非类固醇技术获得的结果相似。LMAN中神经元数量的减少(57%) 一直持续到成体。因此, 成年雌鸟LMAN中的全部神经元数量是雄鸟的4 倍(P<0 001)。推测雌鸟脑中存在较多的神经元数量可能与其在发育和成体期间需维持适当的神经网络有关, 这种神经构筑对于动物种间识别等功能可能具有特殊意义。     

Why does the hoverfly Metasyrphus corollae migrate?

Abstract. 1. Migration pattern and mass appearance of the hoverfly Metasyrphus corollae (F.) (Diptera, Syrphidae) were investigated on the SE coast of Sweden in 1981. In total, 4433 hoverflies, belonging to fifteen species, were collected in water traps during 4–6 August. All but 2.2% were M.corollae , 3282 specimens being caught on 4 August, 1021 on 5 August and thirty-five on 6 August.
2. The sex ratios ( ♂/♀ ) on these dates were 1/1.6, 1/1.3 and 1/1.7. The average dry weights did not significantly differ between sexes (males 4.01 mg, females 4.02 mg).
3. M.corollae females were reproductively immature.
4. Females mostly had Senecio pollen in their guts while males mainly had Rubus pollen. The average number of pollen grains per specimen was 210 for females and 1100 for males.
5. In the summer 1980 aphids were numerous, forming a basis for a high abundance of adult hoverflies (with hibernating larvae) in 1981. Aphid density in 1981 was low and stimuli inducing egglaying were sparse. It is suggested that this pattern of aphid abundance triggers migration in M.corollae and in other aphidophagous species at irregular intervals.     

A new capillariid (nematoda) parasitizing Heterandria bimaculata (Heckel, 1848) (Poeciliidae) in Mexico

A new nematode species, Freitascapillaria moraveci n. sp., is described; it was obtained from specimens recovered from the gall bladder of the 2-spot livebearer Heterandrá bimaculata (Heckel, 1848) from La Antigua River, State of Veracruz, Mexico. The new species is assigned to Freitascapillaria Moravec 1982; it is largely characterized by the posterior end of the males, which is laterally expanded without distinct projections. Freitascapillaria moraveci n. sp. differs from the only other species of the genus, F. maxillosa, by the absence of wing-like cells at the esophago-intestinal junction, which are present in the latter species; stichosoma consists of 30-36 stichocytes (F. moraveci) versus 40-60 stichocytes (F. maxillosa) in both males and females and the presence of a well-developed spicule.     

Hybrid incompatibilities in the parasitic wasp genus Nasonia: negative effects of hemizygosity and the identification of transmission ratio distortion loci

The occurrence of hybrid incompatibilities forms an important stage during the evolution of reproductive isolation. In early stages of speciation, males and females often respond differently to hybridization. Haldane's rule states that the heterogametic sex suffers more from hybridization than the homogametic sex. Although haplodiploid reproduction (haploid males, diploid females) does not involve sex chromosomes, sex-specific incompatibilities are predicted to be prevalent in haplodiploid species. Here, we evaluate the effect of sex/ploidy level on hybrid incompatibilities and locate genomic regions that cause increased mortality rates in hybrid males of the haplodiploid wasps Nasonia vitripennis and Nasonia longicornis. Our data show that diploid F(1) hybrid females suffer less from hybridization than haploid F(2) hybrid males. The latter not only suffer from an increased mortality rate, but also from behavioural and spermatogenic sterility. Genetic mapping in recombinant F(2) male hybrids revealed that the observed hybrid mortality is most likely due to a disruption of cytonuclear interactions. As these sex-specific hybrid incompatibilities follow predictions based on Haldane's rule, our data accentuate the need to broaden the view of Haldane's rule to include species with haplodiploid sex determination, consistent with Haldane's original definition.     

Partial local mate competition and the sex ratio: A study on non-pollinating fig wasps

In many species, mating takes place in temporary patches where only a small number of females produce offspring. In this situation Local Mate Competition (LMC) theory predicts that the optimal sex ratio (defined as proportion males) should become increasingly female biased as the number of females contributing offspring to a patch decreases. However, in a large number of these species, some mating is also likely to occur away from the natal patch (termed partial LMC). In this case the degree of LMC is reduced, and theory predicts a relatively less female biased sex ratio. We tested these two predictions with field data from 17 species of New World non-pollinating fig wasps representing three genera. We present a model which suggests that the average number of females ovipositing in a fruit (i.e. patch) is positively correlated with the proportion of fruit of a given tree species in which that species of wasp occurs. Across species, the overall sex ratio was positively correlated with the proportion of fruit in which that species occurs. Furthermore, the males of some species are wingless, and in these species all mating must take place before females disperse from their natal fruit. In contrast, the males of other species are winged, and in these species mating may also take place away from the natal fruit. Species with winged males had less female biased sex ratios than species with wingless males that occurred in a similar proportion of fruit. Finally, the correlation between sex ratio and the proportion of fruit in which a species occurs was also observed within species when comparing between the fruit crops of different trees. This suggests that individual females facultatively adjust the sex ratio of their offspring in response to variable LMC.     

C-heterochromatin pattern of ten species of tetrigids (Tetrigidae, Orthoptera)

The karyotypes of species belonging to the Tetrigidae is characterised by structural conservatism. The standard chromosome set of T. japonica, T. simulans, T. bolivari, P. meridionalis, U. depressus, and F. robustus consists of 2n = 13 acrocentric chromosomes in males and 2n = 14 in females, with a sex determining mechanism of X0 male and XX female. C-bands distribution often species belonging to 4 genera were studied. Differences in the position of C-bands and number of chiasma between species are discussed.