DeWayne Shoemaker

Wolbachia and the evolution of reproductive isolation between Drosophila recens and Drosophila subquinaria

Endosymbiotic bacteria of the genus Wolbachia are widespread among insects and in many cases cause cytoplasmic incompatibility in crosses between infected males and uninfected females. Such findings have been used to argue that Wolbachia have played an important role in insect speciation. Theoretical models, however, indicate that Wolbachia alone are unlikely to lead to stable reproductive isolation between two formerly conspecific populations. Here we analyze the components of reproductive isolation between Drosophila recens, which is infected with Wolbachia, and its uninfected sister species Drosophila subquinaria. Laboratory pairings demonstrated that gene flow via matings between D. recens females and D. subquinaria males is hindered by behavioral isolation. Matings readily occurred in the reciprocal cross (D. quinaria females × D. recens males), but very few viable progeny were produced. The production of viable hybrids via this route was restored by antibiotic curing of D. recens of their Wolbachia symbionts, indicating that hybrid offspring production is greatly reduced by cytoplasmic incompatibility in the crosses involving infected D. recens males. Thus, behavioral isolation and Wolbachia‐induced cytoplasmic incompatibility act as complementary asymmetrical isolating mechanisms between these two species. In accordance with Haldanes rule, hybrid females were fertile, whereas hybrid males invariably were sterile. Levels of mtDNA variation in D. recens are much lower than in either D. subquinaria or D. falleni, neither of which is infected with Wolbachia. The low haplotype diversity in D. recens is likely due to an mtDNA sweep associated with the spread of Wolbachia. Nevertheless, the existence of several mtDNA haplotypes in this species indicates that Wolbachia have been present as a potential isolating mechanism for a substantial period of evolutionary time. Finally, we argue that although Wolbachia by themselves are unlikely to bring about speciation, they can increase the rate of speciation in insects.

A Y-like social chromosome causes alternative colony organization in fire ants

Intraspecific variability in social organization is common, yet the underlying causes are rarely known. In the fire ant Solenopsis invicta, the existence of two divergent forms of social organization is under the control of a single Mendelian genomic element marked by two variants of an odorant-binding protein gene. Here we characterize the genomic region responsible for this important social polymorphism, and show that it is part of a pair of heteromorphic chromosomes that have many of the key properties of sex chromosomes. The two variants, hereafter referred to as the social B and social b (SB and Sb) chromosomes, are characterized by a large region of approximately 13 megabases (55% of the chromosome) in which recombination is completely suppressed between SB and Sb. Recombination seems to occur normally between the SB chromosomes but not between Sb chromosomes because Sb/Sb individuals are non-viable. Genomic comparisons revealed limited differentiation between SB and Sb, and the vast majority of the 616 genes identified in the non-recombining region are present in the two variants. The lack of recombination over more than half of the two heteromorphic social chromosomes can be explained by at least one large inversion of around 9 megabases, and this absence of recombination has led to the accumulation of deleterious mutations, including repetitive elements in the non-recombining region of Sb compared with the homologous region of SB. Importantly, most of the genes with demonstrated expression differences between individuals of the two social forms reside in the non-recombining region. These findings highlight how genomic rearrangements can maintain divergent adaptive social phenotypes involving many genes acting together by locally limiting recombination.

Species Delimitation: A Case Study in a Problematic Ant Taxon

Species delimitation has been invigorated as a discipline in systematics by an influx of new character sets, analytical methods, and conceptual advances. We use genetic data from 68 markers, combined with distributional, bioclimatic, and coloration information, to hypothesize boundaries of evolutionarily independent lineages (species) within the widespread and highly variable nominal fire ant species Solenopsis saevissima, a member of a species group containing invasive pests as well as species that are models for ecological and evolutionary research. Our integrated approach uses diverse methods of analysis to sequentially test whether populations meet specific operational criteria (contingent properties) for candidacy as morphologically cryptic species, including genetic clustering, monophyly, reproductive isolation, and occupation of distinctive niche space. We hypothesize that nominal S. saevissima comprises at least 4-6 previously unrecognized species, including several pairs whose parapatric distributions implicate the development of intrinsic premating or postmating barriers to gene flow. Our genetic data further suggest that regional genetic differentiation in S. saevissima has been influenced by hybridization with other nominal species occurring in sympatry or parapatry, including the quite distantly related Solenopsis geminata. The results of this study illustrate the importance of employing different classes of genetic data (coding and noncoding regions and nuclear and mitochondrial DNA [mtDNA] markers), different methods of genetic data analysis (tree-based and non-tree based methods), and different sources of data (genetic, morphological, and ecological data) to explicitly test various operational criteria for species boundaries in clades of recently diverged lineages, while warning against over reliance on any single data type (e.g., mtDNA sequence variation) when drawing inferences.

Relaxed selection is a precursor to the evolution of phenotypic plasticity

Phenotypic plasticity allows organisms to produce alternative phenotypes under different conditions and represents one of the most important ways by which organisms adaptively respond to the environment. However, the relationship between phenotypic plasticity and molecular evolution remains poorly understood. We addressed this issue by investigating the evolution of genes associated with phenotypically plastic castes, sexes, and developmental stages of the fire ant Solenopsis invicta. We first determined if genes associated with phenotypic plasticity in S. invicta evolved at a rapid rate, as predicted under theoretical models. We found that genes differentially expressed between S. invicta castes, sexes, and developmental stages all exhibited elevated rates of evolution compared with ubiquitously expressed genes. We next investigated the evolutionary history of genes associated with the production of castes. Surprisingly, we found that orthologs of caste-biased genes in S. invicta and the social bee Apis mellifera evolved rapidly in lineages without castes. Thus, in contrast to some theoretical predictions, our results suggest that rapid rates of molecular evolution may not arise primarily as a consequence of phenotypic plasticity. Instead, genes evolving under relaxed purifying selection may more readily adopt new forms of biased expression during the evolution of alternate phenotypes. These results suggest that relaxed selective constraint on protein-coding genes is an important and underappreciated element in the evolutionary origin of phenotypic plasticity.

Nuclear and mitochondrial genetic structure in two social forms of the fire ant Solenopsis invicta: insights into transitions to an alternate social organization

We describe genetic structure at microgeographical scales in two social forms of the introduced fire ant Solenopsis invicta using nuclear and mitochondrial markers. Fire ant populations in northern Georgia, U.S.A. are structured in fundamentally different ways at the two genomes. Virtually all nuclear genetic variation resides within sampling sites; no differentiation was apparent between the social forms and only minimal differentiation was found among sites in only one social form (the polygyne form, which has nests with numerous queens). In contrast, substantial mtDNA variation exists at all levels; the forms are significantly differentiated and strong divergence was detected among sites in the polygyne form. No mtDNA differentiation was found among sites in the monogyne form (which has nests with a single queen). These results are predicted on the basis of the social biology of the ants. Monogyne queens and males of both forms presumably disperse widely, and polygyne queens commonly mate with monogyne males, thus explaining the uniformity of nuclear allele frequencies across sampling sites and between the forms. Polygyne queens display habits consistent with limited dispersal, and queens of both forms seem unable to establish themselves as reproductives in nests of the alternate form, thus explaining the strong microgeographical structure within the polygyne form and differentiation between the forms found using mtDNA markers. The mtDNA haplotypes are distributed among sites in a manner that suggests a mechanism for the spread of polygyny in introduced fire ants. Monogyne queens apparently mate occasionally with polygyne males and acquire a Mendelian nuclear factor allowing the expression of polygyny in their sexual daughters.

GENETIC STRUCTURE AND EVOLUTION OF A FIRE ANT HYBRID ZONE

Two introduced fire ants, Solenopsis invicta and S. richteri, hybridize over an extensive area in the United States spanning central Mississippi, Alabama, and western Georgia. We studied a portion of this hybrid zone in northwestern Mississippi in detail by sampling ants at many sites along two transects extending across the zone and examining gene frequency and size distributions at a large number of genetic and morphological markers. The distributional patterns at these markers are most consistent with the mosaic hybrid zone model, whereby the distribution of various fire ant genotypes is determined initially by the historical patterns of colonization of newly available habitats. However, these distributional patterns probably do not reflect the equilibrium state of interactions because of the very recent secondary contact of the species (< 60 yr) and the dynamic nature of available nesting habitats in this area. Our data suggest that, with prolonged contact and interaction, differential fitness of various hybrid genotypes due to intrinsic and extrinsic selective factors is important in structuring the hybrid zone. For instance, consistent differential introgression of morphological and genetic markers, combined with previous evidence of differences in developmental stability among genotypes, suggest reduced fitness of hybrids relative to parentals due to intrinsic selection (as may be caused by breakup of parental gene complexes). Furthermore, marked reductions in the occurrence of parental‐like hybrids in areas where the similar parental species is common suggest reduced fitness of these parental‐like hybrids in competition with the parentals (i.e., extrinsic selection). Because the relative roles of such deterministic as well as stochastic forces apparently vary both spatially and temporally, the eventual distribution of the various fire ant genotypes and the fate of the hybrid zone in the United States is difficult to predict.

Evolution of Gene Expression in Fire Ants: The Effects of Developmental Stage, Caste, and Species

Ants provide remarkable examples of equivalent genotypes developing into divergent and discrete phenotypes. Diploid eggs can develop either into queens, which specialize in reproduction, or workers, which participate in cooperative tasks such as building the nest, collecting food, and rearing the young. In contrast, the differentiation between males and females generally depends upon whether eggs are fertilized, with fertilized (diploid) eggs giving rise to females and unfertilized (haploid) eggs giving rise to males. To obtain a comprehensive picture of the relative contributions of gender (sex), caste, developmental stage, and species divergence to gene expression evolution, we investigated gene expression patterns in pupal and adult queens, workers, and males of two species of fire ants, Solenopsis invicta and S. richteri. Microarray hybridizations revealed that variation in gene expression profiles is influenced more by developmental stage than by caste membership, sex, or species identity. The second major contributor to variation in gene expression was the combination of sex and caste. Although workers and queens share equivalent diploid nuclear genomes, they have highly distinctive patterns of gene expression in both the pupal and the adult stages, as might be expected given their extraordinary level of phenotypic differentiation. Overall, the difference in the proportion of differentially expressed genes was greater between workers and males than between workers and queens or queens and males, consistent with the fact that workers and males share neither gender nor reproductive capability. Moreover, between-species comparisons revealed that the greatest difference in gene expression patterns occurred in adult workers, a finding consistent with the fact that adult workers most directly experience the distinct external environments characterizing the different habitats occupied by the two species. Thus, much of the evolution of gene expression in ants may occur in the worker caste, despite the fact that these individuals are largely or completely sterile. Analyses of gene expression evolution revealed a combination of positive selection and relaxation of stabilizing selection as important factors driving the evolution of such genes.

AN UNUSUAL PATTERN OF GENE FLOW BETWEEN THE TWO SOCIAL FORMS OF THE FIRE ANT SOLENOPSIS INVICTA

Uncertainty over the role of shifts in social behavior in the process of speciation in social insects has stimulated interest in determining the extent of gene flow between conspecific populations differing in colony social organization. Allele and genotype frequencies at 12 neutral polymorphic protein markers, as well as the numbers of alleles at the sex‐determining locus (loci), are shown here to be consistent with significant ongoing gene flow between two geographically adjacent populations of Solenopsis invicta that differ in colony queen number. Data from a thirteenth protein marker that is under strong differential selection in the two social forms confirm that such gene flow occurs. Data from this selected locus, combined with knowledge of the reproductive biology of the two social forms, further suggest that interform gene flow is largely unidirectional and mediated through males only. This unusual pattern of gene flow results from the influence of the unique social enviroments of the two forms on the behavior of workers and on the reproductive physiology of sexuals.

Mitochondrial genome evolution in fire ants (Hymenoptera: Formicidae).

BackgroundComplete mitochondrial genome sequences have become important tools for the study of genome architecture, phylogeny, and molecular evolution. Despite the rapid increase in available mitogenomes, the taxonomic sampling often poorly reflects phylogenetic diversity and is often also biased to represent deeper (family-level) evolutionary relationships.ResultsWe present the first fully sequenced ant (Hymenoptera: Formicidae) mitochondrial genomes. We sampled four mitogenomes from three species of fire ants, genus Solenopsis, which represent various evolutionary depths. Overall, ant mitogenomes appear to be typical of hymenopteran mitogenomes, displaying a general A+T-bias. The Solenopsis mitogenomes are slightly more compact than other hymentoperan mitogenomes (~15.5 kb), retaining all protein coding genes, ribosomal, and transfer RNAs. We also present evidence of recombination between the mitogenomes of the two conspecific Solenopsis mitogenomes. Finally, we discuss potential ways to improve the estimation of phylogenies using complete mitochondrial genome sequences.ConclusionsThe ant mitogenome presents an important addition to the continued efforts in studying hymenopteran mitogenome architecture, evolution, and phylogenetics. We provide further evidence that the sampling across many taxonomic levels (including conspecifics and congeners) is useful and important to gain detailed insights into mitogenome evolution. We also discuss ways that may help improve the use of mitogenomes in phylogenetic analyses by accounting for non-stationary and non-homogeneous evolution among branches.

Estimation of the number of founders of an invasive pest insect population: the fire ant Solenopsis invicta in the USA

Determination of the number of founders responsible for the establishment of invasive populations is important for developing biologically based management practices, predicting the invasive potential of species, and making inferences about ecological and evolutionary processes. The fire ant Solenopsis invicta is a major invasive pest insect first introduced into the USA from its native South American range in the mid-1930s. We use data from diverse genetic markers surveyed in the source population and the USA to estimate the number of founders of this introduced population. Data from different classes of nuclear markers (microsatellites, allozymes, sex-determination locus) and mitochondrial DNA are largely congruent in suggesting that 9–20 unrelated mated queens comprised the initial founder group to colonize the USA at Mobile, Alabama. Estimates of founder group size based on expanded samples from throughout the southern USA were marginally higher than this, consistent with the hypothesis of one or more secondary introductions of the ant into the USA. The rapid spread and massive population build-up of introduced S. invicta occurred despite the loss of substantial genetic variation associated with the relatively small invasive propagule size, a pattern especially surprising in light of the substantial genetic load imposed by the loss of variation at the sex-determination locus.