Biomedical subjects

Gerald S Wilkinson

Publications and source records attributed to Gerald S Wilkinson.

12 recordsLinked to original sources

PubMed2006

Correlated evolution between hearing sensitivity and social calls in bats.

Echolocating bats are auditory specialists, with exquisite hearing that spans several octaves. In the ultrasonic range, bat audiograms typically show highest sensitivity in the spectral region of their species-specific echolocation calls. Well-developed hearing in the audible range has been commonly attributed to a need to detect sounds produced by prey. However, bat pups often emit isolation calls with low-frequency components that facilitate mother-young reunions. In this study, we examine whether low-frequency hearing in bats exhibits correlated evolution with (i) body size; (ii) high-frequency hearing sensitivity or (iii) pup isolation call frequency. Using published audiograms, we found that low-frequency hearing sensitivity is not dependent on body size but is related to high-frequency hearing. After controlling for high-frequency hearing, we found that low-frequency hearing exhibits correlated evolution with isolation call frequency. We infer that detection and discrimination of isolation calls have favoured enhanced low-frequency hearing because accurate parental investment is critical: bats have low reproductive rates, non-volant altricial young and must often identify their pups within large crèches.

Animals↗

PubMed2006

Mating system and brain size in bats.

The contribution of sexual selection to brain evolution has been little investigated. Through comparative analyses of bats, we show that multiple mating by males, in the absence of multiple mating by females, has no evolutionary impact on relative brain dimension. In contrast, bat species with promiscuous females have relatively smaller brains than do species with females exhibiting mate fidelity. This pattern may be a consequence of the demonstrated negative evolutionary relationship between investment in testes and investment in brains, both metabolically expensive tissues. These results have implications for understanding the correlated evolution of brains, behaviour and extravagant sexually selected traits.

Animals↗

PubMed2005

Genetic linkage between a sexually selected trait and X chromosome meiotic drive.

Previous studies on the stalk-eyed fly, Cyrtodiopsis dalmanni, have shown that males with long eye-stalks win contests and are preferred by females, and artificial selection on male relative eye span alters brood sex-ratios. Subsequent theory proposes that X-linked meiotic drive can catalyse the evolution of mate preferences when drive is linked to ornament genes. Here we test this prediction by mapping meiotic drive and quantitative trait loci (QTL) for eye span. To map QTL we genotyped 24 microsatellite loci using 1228 F2 flies from two crosses between lines selected for long or short eye span. The crosses differed by presence or absence of a drive X chromosome, X(D), in the parental male. Linkage analysis reveals that X(D) dramatically reduces recombination between X and X(D) chromosomes. In the X(D) cross, half of the F2 males carried the drive haplotype, produced partially elongated spermatids and female-biased broods, and had shorter eye span. The largest QTL mapped 1.3cM from drive on the X chromosome and explained 36% of the variation in male eye span while another QTL mapped to an autosomal region that suppresses drive. These results indicate that selfish genetic elements that distort the sex-ratio can influence the evolution of exaggerated traits.

Animals↗

PubMed2005

Genetic divergence does not predict change in ornament expression among populations of stalk-eyed flies.

Stalk-eyed flies (Diptera: Diopsidae) possess eyes at the ends of elongated peduncles, and exhibit dramatic variation in eye span, relative to body length, among species. In some sexually dimorphic species, evidence indicates that eye span is under both intra- and intersexual selection. Theory predicts that isolated populations should evolve differences in sexually selected traits due to drift. To determine if eye span changes as a function of divergence time, 1370 flies from 10 populations of the sexually dimorphic species, Cyrtodiopsis dalmanni and Cyrtodiopsis whitei, and one population of the sexually monomorphic congener, Cyrtodiopsis quinqueguttata, were collected from Southeast Asia and measured. Genetic differentiation was used to assess divergence time by comparing mitochondrial (cytochrome oxidase II and 16S ribosomal RNA gene fragments) and nuclear (wingless gene fragment) DNA sequences for c. five individuals per population. Phylogenetic analyses indicate that most populations cluster as monophyletic units with up to 9% nucleotide substitutions between populations within a species. Analyses of molecular variance suggest a high degree of genetic structure within and among the populations; > 97% of the genetic variance occurs between populations and species while < 3% is distributed within populations, indicating that most populations have been isolated for thousands of years. Nevertheless, significant change in the allometric slope of male eye span on body length was detected for only one population of either dimorphic species. These results are not consistent with genetic drift. Rather, relative eye span appears to be under net stabilizing selection in most populations of stalk-eyed flies. Given that one population exhibited dramatic evolutionary change, selection, rather than genetic variation, appears to constrain eye span evolution.

Animals↗

PubMed2005

Rapid evolution of postzygotic reproductive isolation in stalk-eyed flies.

We test the relative rates of evolution of pre- and postzygotic reproductive isolation using eight populations of the sexually dimorphic stalk-eyed flies Cyrtodiopsis dalmanni and C. whitei. Flies from these populations exhibit few morphological differences yet experience strong sexual selection on male eyestalks. To measure reproductive isolation we housed one male and three female flies from within and between these populations in replicate cages and then recorded mating behavior, sperm transfer, progeny production, and hybrid fertility. Using a phylogeny based on partial sequences of two mitochondrial genes, we found that premating isolation, postmating isolation prior to hybrid eclosion, and female hybrid sterility evolve gradually with respect to mitochondrial DNA sequence divergence. In contrast, male hybrid sterility evolves much more rapidly-at least twice as fast as any other form of reproductive isolation. Hybrid sterility, therefore, obeys Haldane's rule. Although some brood sex ratios were female biased, average brood sex ratio did not covary with genetic distance, as would be expected if hybrid inviability obeyed Haldane's rule. The likelihood that forces including sexual selection and intra- and intergenomic conflict may have contributed to these patterns is discussed.

Animals↗

PubMed2004

Microsatellite variation among divergent populations of stalk-eyed flies, genus Cyrtodiopsis.

Microsatellite primers are often developed in one species and used to assess neutral variability in related species. Such analyses may be confounded by ascertainment bias (i.e. a decline in amplification success and allelic variability with increasing genetic distance from the source of the microsatellites). In addition, other factors, such as the size of the microsatellite, whether it consists of perfect or interrupted tandem repeats, and whether it is autosomal or X-linked, can affect variation. To test the relative importance of these factors on microsatellite variation, we examine patterns of amplification and allelic diversity in 52 microsatellite loci amplified from five individuals in each of six populations of Cyrtodiopsis stalk-eyed flies that range from 2.2 % to 11.2% mitochondrial DNA sequence divergence from the population used for microsatellite development. We find that amplification success and most measures of allelic diversity declined with genetic distance from the source population, in some cases an order of magnitude faster than in birds or mammals. The median and range of the repeat array length did not decline with genetic distance. In addition, for loci on the X chromosome, we find evidence of lower observed heterozygosity compared with loci on autosomes. The differences in variability between X-linked and autosomal loci are not adequately explained by differences in effective population sizes of the chromosomes. We suggest, instead, that periodic selection events associated with X-chromosome meiotic drive, which is present in many of these populations, reduces X-linked variation.

Animals↗

PubMed2004

Sperm survival in female stalk-eyed flies depends on seminal fluid and meiotic drive.

Sperm competition is common in many insect species; however, the mechanisms underlying differences in sperm precedence are not well understood. In the stalk-eyed fly, Cyrtodiopsis whitei (Diptera, Diopsidae), sperm precedence is influenced by the presence of sex chromosome meiotic drive. When drive-carrying males compete with non-driving males for fertilizations within a female, the number of progeny sired by drive males is significantly fewer than predicted by sperm mixing alone. Thus, drive males apparently suffer not only a reduction in the number of viable sperm produced, but also a reduction in sperm competitive ability. In this study, we manipulated the amount and source of seminal fluid and sperm received by females by interrupting copulations before sperm, but after seminal fluid, was transferred. We find that seminal fluid from another male influences the number of progeny sired by a drive-carrying male when both males mate with the same female. Sperm viability staining reveals that sperm from drive males are incapacitated by seminal fluid from other males within the female reproductive tract. These results suggest that multiple mating by females enables seminal fluid products to interact differentially with sperm and may reduce the transmission advantage of the drive chromosome.

Analysis of Variance↗

PubMed2003

Phylogeography of sex ratio and multiple mating in stalk-eyed flies from southeast Asia.

The factors maintaining sex chromosome meiotic drive, or sex ratio (SR), in natural populations remain uncertain. Coevolution between segregation distortion and modifiers should produce transient SR distortion while selection can result in a stable polymorphism. We hypothesize that if SR is maintained by selection, then phylogenetically related populations should exhibit similar SR frequency and intensity. Furthermore, when drive is present, females should mate with multiple males more often both to insure fertility and to increase the probability of producing male progeny. In this paper we report on variation in SR frequency and multiple mating among seven populations and three species of stalk-eyed flies, genus Cyrtodiopsis, from southeast Asia. Using a phylogenetic hypothesis based on 1100 bp of mtDNA sequence we find that while sex chromosome meiotic drive is present in all populations of C. whitei and C. dalmanni, the frequency and intensity of drive only differs between populations or species with greater than 4.8% sequence divergence. The frequency of females mating with multiple males is higher in populations with SR. In addition, SR males mate less often, possibly to compensate for sperm depletion. Our results suggest that sex chromosome drive is maintained by balancing selection in populations of C. whitei and C. dalmanni. Nevertheless, coevolution between drive and suppressors deserves further study.

Animals↗

PubMed2003

Male dominance, paternity, and relatedness in the Jamaican fruit-eating bat (Artibeus jamaicensis).

We analysed variation at 14 nuclear microsatellite loci to assess the genetic structure, relatedness, and paternity of polygynous Jamaican fruit-eating bats. A total of 84 adults captured in two caves exhibited little genetic differentiation between caves (FST = 0.008). Average relatedness among adult females in 10 harem groups was very low (R = 0.014 +/- 0.011), providing no evidence of harem structure. Dominant and subordinate males shared paternity in large groups, while dominant and satellite males shared paternity in smaller groups. However, our results suggest that male rank influences paternity. Dominant males fathered 69% of 40 offspring, followed by satellite (22%) and subordinate males (9%). Overall adult male bats are not closely related, however, in large harem groups we found that subordinate and dominant males exhibited relatedness values consistent with a father-offspring relationship. Because dominant and subordinate males also sired all the pups in large groups, we propose that their association provides inclusive fitness to them.

Alleles↗

PubMed2003

Phylogenetic analysis of correlation structure in stalk-eyed flies (Diasemopsis, Diopsidae).

Morphological divergence among species may be constrained by the pattern of genetic variances and covariances among traits within species. Assessing the existence of such a relationship in nature requires information on the stability of intraspecific correlation and covariance structure and the correspondence of this structure to the pattern of evolutionary divergence within a lineage. Here, we investigate these issues for nine morphological traits and 15 species of stalk-eyed flies in the genus Diasemopsis. Within-species matrices for these traits were generated from phenotypic data for all the Diasemopsis species and from genetic data for a single Diasemopsis species, D. dubia. The among-species pattern of divergence was assessed by calculating the evolutionary correlations for all pairwise combinations of the morphological traits along the phylogeny of these species. Comparisons of intraspecific matrices reveal significant similarity among all species in the phenotypic correlations matrices but not the covariance matrices. In addition, the differences in correlation structure that do exist among species are not related to their phylogenetic placement or change in the means of the traits. Comparisons of the phenotypic and phylogenetic matrices suggest a strong relationship between the pattern of evolutionary change among species and both the intraspecific correlation structure and the stability of this structure among species. The phenotypic and the phylogenetic matrices are significantly similar, and pairs of traits whose intraspecific correlations are more stable across taxa exhibit stronger coevolution on the phylogeny. These results suggest either the existence of strong constraints on the pattern of evolutionary change or a consistent pattern of correlated selection shaping both the phenotypic and phylogenetic matrices. The genetic correlation structure for D. dubia, however, does not correspond with patterns found in the phenotypic and phylogenetic data. Possible reasons for this disagreement are discussed.

Animals↗

PubMed2002

The long and short of sperm polymorphisms in insects.

Production of more than one morphological type of sperm in a common testis has been documented for a variety of invertebrates, including gastropods, spiders, centipedes, and insects. This unusual phenomenon is difficult to explain by current theory, particularly since available evidence indicates that one sperm type is often incapable of effecting fertilization. In this review we critically examine evidence on the distribution and development of sperm heteromorphisms among insects in light of competing hypotheses for the evolutionary origin, maintenance, and function of a non-fertilizing class of sperm. To date, no single hypothesis, including alternatives which assume non-fertilizing sperm are non-adaptive, or that they provision, facilitate, or compete with fertilizing sperm, has received strong empirical support by any group of insects. The diversity of sperm heteromorphisms suggests that non-fertilizing sperm may have different functions in different clades or even serve multiple functions within a clade. We suggest that insight could be gained from (1) new models for the evolution of sperm polymorphism, (2) comparative studies that focus on multiple traits simultaneously (e.g. sperm number, proportion, length, and remating rate) and utilize clades in which more than one gain or loss of sperm heteromorphism has been documented (e.g. Pentatomidae, Carabidae, or Diopsidae), and (3) experimental studies that exploit individual variation or directly manipulate the composition of the male ejaculate.

Animals↗

PubMed2002

Life history, ecology and longevity in bats.

The evolutionary theory of aging predicts that life span should decrease in response to the amount of mortality caused by extrinsic sources. Using this prediction, we selected six life history and ecological factors to use in a comparative analysis of longevity among 64 bat species. On average, the maximum recorded life span of a bat is 3.5 times greater than a non-flying placental mammal of similar size. Records of individuals surviving more than 30 years in the wild now exist for five species. Univariate and multivariate analyses of species data, as well as of phylogenetically independent contrasts obtained using a supertree of Chiroptera, reveal that bat life span significantly increases with hibernation, body mass and occasional cave use, but decreases with reproductive rate and is not influenced by diet, colony size or the source of the record. These results are largely consistent with extrinsic mortality risk acting as a determinant of bat longevity. Nevertheless, the strong association between life span and both reproductive rate and hibernation also suggests that bat longevity is strongly influenced by seasonal allocation of non-renewable resources to reproduction. We speculate that hibernation may provide a natural example of caloric restriction, which is known to increase longevity in other mammals.

Animals↗