Fitness and life-history traits of the two major mitochondrial DNA haplotypes of Drosophila subobscura

Mitochondrial DNA restriction site analyses on natural populations of Drosophila subobscura have proved the existence of two common, coexisting haplotypes (I and II), as well as a set of less frequent ones derived from them. To explain this distribution, experiments to date point practically to all possible genetic mechanisms being involved in the changes of gene frequencies (cytonuclear coadaptation, direct natural selection on mtDNA and genetic drift). In an attempt to find differences that help to understand the dynamics of these haplotypes and to detect the effect of selection, we measured certain fitness components and life-history traits (egg-larva and larva-adult viabilities and developmental times, longevity, resistance to desiccation and optimal density) of the two main haplotypes I and II when maintained in laboratory population cages. As a general trend, haplotype II showed a higher net fitness than haplotype I, which explains the superiority of haplotype II over haplotype I in experimental populations but not their coexistence in nature, where additional factors must be considered.     

Female discrimination thresholds frequently exceed local male display variation: implications for mate choice dynamics and sexual selection

Among the factors that can influence female mate choice decisions is the degree to which females differentiate among similar displays: as differences decrease, females are expected to eventually stop discriminating. This discrimination threshold, in conjunction with the magnitude of male trait variation females regularly encounter while making mate choice decisions, may have important consequences for sexual selection. If local display variation is above the discrimination threshold, female preferences should translate into higher mating success for the more attractive male. But if display variation is frequently below the threshold, the resulting increased pattern of random mating may obscure the existence of female mate choice. I investigated the interplay between female discrimination and male display variation in green treefrogs (Hyla cinerea) and found that call trait differences between nearest neighbour males were frequently smaller than what females are expected to discriminate. This finding has two important consequences for our understanding of sexual selection in the wild: first, low display variation should weaken the strength of selection on male display traits, but the direction of selection should mirror the one predicted from females choice trials. Second, caution is needed when interpreting data on realized mating success in the wild: a pattern of random mating with respect to male display traits does not always mean that female preferences are weak or that conditions are too challenging for females to express their preferences. Rather, insufficient display variation can generate the same pattern.     

HLA and mate choice in humans.

Evidence from studies in rodents suggests that mate selection is influenced by major-histocompatibility-complex haplotypes, with preferences for dissimilar partners. This study was initiated to determine whether avoidance of a mate with the same HLA haplotype as one's own might be occurring in the Hutterites, a North American reproductive isolate of European ancestry, notable for their large sibships, communal lifestyle, and limited number of five-locus HLA haplotypes (HLA-A, -B, -C, -DR, and -DQ). HLA haplotypes were known for 411 Hutterite couples. The number of couples expected to match for a haplotype was calculated in two ways: first, from population genotype frequencies, with account being taken of the nonrandom mating pattern with respect to colony lineages, and, second, from computer simulations using conservative founder assumptions and the exact genealogy of the 411 couples. We observed fewer matches for HLA haplotypes between spouses than expected (first method, P = .005; second method, P = .020-.067). Among couples who did match for a haplotype, the matched haplotype was inherited from the mother in 29 cases and from the father in 50 cases (P = .018). These results are consistent with the conclusion that Hutterite mate choice is influenced by HLA haplotypes, with an avoidance of spouses with haplotypes that are the same as one's own.     

FERTILITY SELECTION ON A DISCRETE FLORAL POLYMORPHISM IN CLARKIA (ONAGRACEAE)

This study investigated fertility selection on a flower petal pigmentation polymorphism in Clarkia gracilis ssp. sonomensis. Natural populations are typically composed of nearly 100% spotted-petal plants, although rare populations contain a majority of unspotted plants. I compared fitness values for the two morphs using a simple fertility model to estimate selection for experimental arrays of plants placed into existing populations of different phenotypic frequencies. Both male and female reproductive success were estimated as well as the pattern of mating among phenotypes. Although the separate fitness components varied from no differences to a strong advantage for spotted plants, for every situation the selection calculations predicted an increase in the frequency of the spotted allele. Pollinator behavior and postpollination mechanisms may be responsible for the fitness differences. The apparent inability of the unspotted allele to spread though most natural populations is consistent with its selective disadvantage in this study.     

Associations among cytoplasmic molecular markers,gender, and components of fitness in Silene vulgaris,a gynodioecious plant

It has been suggested that the dynamics of chloroplast DNA (cpDNA) or mitochondrial DNA (mtDNA) genetic markers used in studies of plant populations could be influenced by natural selection acting elsewhere in the genome. This could be particularly true in gynodioecious plants if cpDNA or mtDNA genetic markers are in gametic disequilibrium with genes responsible for sex expression. In order to investigate this possibility, a natural population of the gynodioecious plant Silene vulgaris was used to study associations among mtDNA haplotype, cpDNA haplotype, sex and some components of fitness through seed. Individuals were sampled for mtDNA and cpDNA haplotype as determined using restriction fragment length polymorphism (RFLP) methods, sex (female or hermaphrodite), fruit number, fruit set, seeds/fruit and seed germination. The sex of surviving germinating seeds was also noted. All individuals in the population fell into one of two cytoplasmic categories, designated haplotypes f and g by a unique electrophoretic signature in both the mtDNA and cpDNA. The subset of the population carrying haplotype g included a significantly higher proportion of females when compared with the sex ratio of the subset carrying the f haplotype. Haplotype g had a significantly higher fitness when measured by fruit number, fruit set and seeds/fruit, whereas haplotype f had significantly higher fitness when measured by seed germination. Offspring of individuals carrying haplotype g included a significantly greater proportion of females when compared with offspring of individuals carrying the f haplotype. Other studies of gynodioecious plants have shown that females generally have higher fitness through seed than hermaphrodites, but in this study not all fitness differences between haplotypes could be predicted from differences in haplotype-specific sex ratio alone. Rather, some differences in haplotype-specific fitness were due to differences in fitness between individuals of the same sex, but carrying different haplotypes. The results are discussed with regard to the potential for hitchhiking selection to influence the dynamics of the noncoding regions used to designate the cpDNA and mtDNA haplotypes.     

Spatio-genetic population structure in mustached tamarins, Saguinus mystax

Dispersal and philopatry influence gene flow and thus the spatio-genetic structure within and between populations. In callitrichids the flexible social and mating system corresponds with a variable migration pattern where both sexes might be philopatric or might disperse. We investigated the relationship between the spatio-genetic structure and migration patterns in a population of mustached tamarins, Saguinus mystax. Using the rapidly evolving hypervariable region I (HVI) of the mitochondrial control region and 11 microsatellite markers we detected a high variation (HVI: 16 haplotypes in 69 individuals; microsatellites: H(O) = 0.75, average: 7.45 alleles/locus), with mating partners usually not sharing the same haplotype, indicating that matings are generally between partners that are not closely related. Similar high variance of haplotype differences for male-male and female-female pairs, along with a slightly higher number of haplotype differences in males show that both sexes habitually migrate. Spatial analyses suggest that females usually migrate longer distances, corresponding to very limited breeding positions for females in a polyandrous social mating system.     

Scaling of Size and Dimorphism in Primates II: Macroevolution

Previous researchers found positive scaling of body size and sexual size dimorphism (SSD) in primates, known as Rensch's rule. The pattern is present in Haplorhini, but absent in Strepsirhini. I found that positive evolutionary correlations between size and SSD drive positive scaling relationships within Haplorhini as a whole and Platyrrhini, Cercopithecinae, Colobinae, and Hominoidea individually at the generic level and higher, but that evolutionary correlations within genera in these clades are often nonsignificant or negative. I suggest that positive evolutionary correlations result from greater change in male than in female size, usually because of sexual selection acting on polygynous populations. I suggest that negative evolutionary correlations result from greater change in female size, owing to either natural selection or, in Callitrichidae, sexual selection acting on polyandrous populations. The high incidence of negative evolutionary correlations within Haplorhini suggests a relatively large influence of natural selection on SSD, at least with regard to differences in SSD between congeners. I propose two possible explanations for the difference in intrageneric and supergeneric evolutionary patterns: 1) natural selection is a relatively weak force for modifying SSD and has a noticeable effect only when one compares related species experiencing similar levels of sexual selection, and 2) natural selection is a relatively strong force for modifying SSD but is less likely than sexual selection to affect higher level taxonomic comparisons noticeably because of the cumulative effect over time of marginal differences in mortality rates of these two types of selection. I discuss types of data required to test these explanations and implications for reconstructing fossil behavior.     

Female multiple mating in wild and laboratory populations of the two-spot ladybird, Adalia bipunctata

Female mating rate is an important variable for understanding the role of females in the evolution of mating systems. Polyandry influences patterns of sexual selection and has implications for sexual conflict over mating, as well as for wider issues such as patterns of gene flow and levels of genetic diversity. Despite this, remarkably few studies of insects have provided detailed estimates of polyandry in the wild. Here we combine behavioural and molecular genetic data to assess female mating frequency in wild populations of the two-spot ladybird, Adalia bipunctata (Coleoptera: Coccinellidae). We also explore patterns of sperm use in a controlled laboratory environment to examine how sperm from multiple males is used over time by females, to link mating with fertilization. We confirm that females are highly polyandrous in the wild, both in terms of population mating rates (approximately 20% of the population found in copula at any given time) and the number of males siring offspring in a single clutch (three to four males, on average). These patterns are consistent across two study populations. Patterns of sperm use in the laboratory show that the number of mates does not exceed the number of fathers, suggesting that females have little postcopulatory influence on paternity. Instead, longer copulations result in higher paternity for males, probably due to the transfer of larger numbers of sperm in multiple spermatophores. Our results emphasize the importance of combining field and laboratory data to explore mating rates in the wild.     

Sexual selection on multivariate phenotype in wild and mass-reared Ceratitis capitata (Diptera: Tephritidae)

Males with a larger thorax and narrower face were found to be favoured by directional sexual selection in a previous field cage experiment where Ceratitis capitata males from the Seib 6-96 genetic sexing strain competed with wild males from Alto Valle (Patagonia) for the possession of wild females. Targets of sexual selection, however, might differ between wild and laboratory females as a response to adaptation to mass-rearing conditions. To evaluate possible divergences on the targets of sexual selection as a by-product of adaptation to mass-rearing conditions, field cage tests were performed with both wild and laboratory females. To avoid possible bias due to correlation among the measured traits (eye length [EL], face width [FW], head width [HW], and thorax length [TL]), a multivariate analysis was applied. Consistent with the previous experiment, the results indicated that TL and FW are probable targets of directional sexual selection independently of female strain. However, laboratory females were less selective than wild ones. Additionally, correlational sexual selection was detected acting on the multivariate phenotype. The effects of correlational selection overlap with those of directional selection on each single trait. The analysis of mating pair characteristics showed patterns that do not match the expectations for a random mating system. The current analysis indicates that during mating pair formation two processes overlap. On the one hand, sexual selection favours males with larger size (TL) and narrower faces (FW). This effect occurs in both wild and laboratory females. In addition, assortative mating based on both phenotype and origin was also observed.     

Female reproductive success and the evolution of mating-type frequencies in tristylous populations

In tristylous populations, mating-type frequencies are governed by negative frequency-dependent selection typically resulting in equal morph ratios at equilibrium. However, Narcissus triandrus generally exhibits long-styled (L)-biased populations with a deficiency of the mid-styled (M)-morph. Here we used a pollen-transfer model and measurements of female fertility in natural populations to investigate whether these uneven morph ratios were associated with variation in female reproductive success. Our theoretical analysis demonstrated that morph ratio bias can result from maternal fitness differences among the morphs, and that these effects were magnified by asymmetrical mating. In nine out of 15 populations of N. triandrus, seed set differed significantly among the morphs, but pollen limitation occurred in only two of 11 populations investigated. Average seed set of the M-morph was positively associated with its frequency in populations. Flower size was negatively correlated with the seed set of the M-morph. Our results suggest that interactions between mating patterns and female fertility are responsible for variation in morph frequencies and loss of the M-morph from tristylous populations of N. triandrus.