What can ants tell us about speciation? 

Ants are among the most diverse and ecologically successful organisms on the planet, yet little is known about how new ant species arise. In this blog post by Sara E. Miller, talks about how Heidbreder and colleagues synthesized current knowledge on the evolutionary processes driving ant diversification in their review article, “Speciation in ants: unlocking ant diversity to study speciation (Hymeno­ptera: Formicidae)”. By examining both reproductive barriers and broad macroevolutionary patterns, the authors highlight why can be a powerful model system for studying speciation. This work underscores an exciting opportunity to uncover the genetic and ecological mechanisms that generate the extraordinary global diversity of ants.

Edit by Purbayan Ghosh and Salvatore Brunetti

A review by Sara E. Miller

If there is one universal truth in biology, it is that there are a lot of ants. The family Formicidae contains over 14,000 described species, spread almost everywhere on Earth, contributing to many diverse ecosystem functions- from soil engineers to pest controllers. While some ant species provides positive economic impacts to nearby communities, some might have negative impacts. Despite this diversity in both species number and species function, the processes that generate new ant species have been surprisingly neglected as an area of study. In their recent review paper, “Speciation in ants: unlocking ant diversity to study speciation (Hymeno­ptera: Formicidae)”, Heidbreder and colleagues explore what we know about speciation in ants and discuss the ways in which ants are uniquely suited for informing our understanding of speciation.

New species form when reproductive isolating barriers develop, preventing gene flow and leading to two genetically distinct populations. Prior studies in ants have identified multiple types of prezygotic isolating barriers – those barriers that prevent mating or the formation of zygotes. Modifications of sex pheromones or cuticular hydrocarbon (CHC) used as mating signals can decrease the attractiveness of individuals to members of the opposite species. Similarly, changes to the timing of when sexual offspring are produced, nuptial flights, or habitat preferences can prevent mating between species and cause reproductive isolation. Although prezygotic isolating barriers have been the subject of numerous studies, given the diversity of ants, there is still much we don’t know about these barriers. The authors suggest that a more systematic study of sister species using a standardized framework for classifying reproductive isolation would increase our understanding of the importance of each type of barrier, and lead to more generalized conclusions about possible drivers of reproductive isolation in ants.

Another promising area of study in ant speciation is regarding the formation of a specific type of postzygotic reproductive isolating barrier known as Bateson-Dobzhansky-Muller incompatibilities (BDMIs). BDMIs occur when alleles from two parental species are incompatible in hybrids, leading to reduced fitness or even inviability of hybrid offspring. BDMIs are hypothesized to be largely caused by recessive alleles. In ants and other haplodiploid species, deleterious recessive alleles can be masked in diploid females but are immediately exposed to selection in haploid males. This has three consequences for the study of speciation in ants. First, BDMIs may develop more slowly in haplodiploid species as many deleterious recessive alleles will be removed by selection against males before they can build up in parental populations. Second, hybridization has different impacts for male versus female offspring. In a cross between a female of species A and a male of species B, all female offspring will be F1 hybrids while all males will be species A. Hybrid males can only be produced one generation later, as sons of the F1 females. Thus, male hybrids always lag a generation behind female hybrids, and furthermore, male hybrids cannot be produced if the F1 females are sterile. These features may explain, at least in part, some of the remarkably unusual characteristics observed in many hybridizing ant species. A third implication of BDMIs in haplodiploids is that they offer a unique opportunity to rapidly identify and map reproductive barrier loci by comparing allele frequencies between haploid males and diploid females. Alleles associated with barrier loci should be missing in populations of hybrid males but present in populations of hybrid females. This is a promising area for future studies to identify barrier loci aka “speciation genes” and to increase our understanding of how these barrier loci form in the genome.

At the macroevolutionary level, Heidbreder and colleagues also describe several promising areas of future study for identifying broader patterns in diversification across the ants. Biotic interactions among species have been identified as drivers of diversity in other taxa. Do co-evolutionary relationships such as those between ants and plants, fungus and farming ants, or aphids and aphid farming ants lead to increased rates of speciation? Or is ant diversification better described as an adaptive radiation in which rapid diversification only occurs after species expand into new habitats, or develop novel traits that allow them to access new resources?

While speciation in ants may not yet have received the attention it deserves, now is an ideal time to explore it. The recent development of extensive ant trait databases combined with the ever-expanding availability of new genomic resources for ants provides an easily tractable system for studying reproductive isolation at the microevolutionary level, and broader patterns of diversification at the macroevolutionary level.