Reproductive barriers and speciation in marine isopods of the Jaera albifrons group – ISOBAR

The first research axis, focused on behavioral isolation, combines three approaches:
an experimental study of mate choice within the Jaera albifrons species complex
an experimental study of sexual selection within one species of the complex
an analysis of mate choice in a population that shows an unusual pattern of hybridization

With the second research axis, we will look at post-zygotic isolating barriers using two approaches:
an experimental study of viability and fertility of interspecific hybrids from populations with different levels of hybridization occurring in the wild.
a study of the genomic architecture of species divergence using RAD-sequencing, with a particular focus on the role of sex-chromosomes (ZW).

The beginning of the project has been dominated by methodological set-up and development (lab-rearing conditions, feasibility of experimental crosses, and development of molecular tools such as microsatellite markers and RAD-seq).
Our most interesting result so far has been to find a series of populations with a mixture of two species with different levels of hybridization. We also found that different regions of the genome will most likely be differently affected by inter-specific gene flow (as shown by the peculiar behavior of one of our microsatellite markers). This result is promising for our genome scan analysis, scheduled for the next period using RAD-sequencing.

For the next period we will focus mainly on the following objectives:
-  extend our analysis of the hybrid zone and comparison with non-hybridizing populations
-  estimate quantitatively the strength of isolating barriers between our two hybridizing species
-  realize a genome scan of the differentiation between these species (comparing populations with vs without hybridization)

A technical primer note presenting the development of microsatellite loci for our species complex was submitted for consideration in BMC Research Notes.

This proposal focuses on speciation. Life forms are extraordinarily ramified, and while we recognize the biological species as a fundamental level of organization, we are left with many questions regarding the processes by which new species emerge.
Evolutionary biology has forged a comprehensive view of the barriers that prevent gene flow between species. We are now struggling to understand the mechanisms behind these isolating barriers, with the overarching goal of inferring how they appear and develop. We focus here on two isolating barriers that were shown to be decisive for the diversification of animals. The first one is behavioral isolation, which results from mate choice preferences for conspecifics during sexual reproduction. One pertaining issue in this context is the link between such mate choice processes between species with sexual selection processes occurring within species. The rare study systems where mate choice mechanisms have been studied at these two levels simultaneously have yielded cutting-edge insights into the evolution of sexual isolation. One needs today to expand this type of approaches by looking at species complexes showing some variance in the barriers to gene flow between pairs of species and where sexual isolation and sexual selection can be analyzed concurrently.
The second isolating barrier that we will be looking at is postzygotic genetic incompatibilities. When premating behavioral isolation is absent or imperfect, gene flow between species may still be limited through genetic incompatibilities if they reduce the viability and/or fertility of hybrids. Theoretical research has proposed straightforward hypotheses for explaining the evolution of postzygotic isolation, and in some cases empirical research has been as far as identifying actual genes involved in incompatibilities. However, such research is heavily biased towards the Drosophila genus, which has been a frustration for determining the generality of the conclusions obtained. Furthermore, it was shown that sex chromosomes have a particular role to play in the accumulation and expression of such incompatibilities (and possibly also in premating behavioral isolation). While this has been fairly well described in male heterogametic (XY) taxa, we need to push forward empirical research on female heterogametic (ZW) species to broaden our understanding of the role of sex chromosomes in speciation.
In this project we propose a concurrent study of behavioral isolation and postzygotic isolation in a complex of species showing some variance in the barriers to gene flow between species. We will use the Jaera albifrons group (marine isopods) as our study system, combining breeding experiments and molecular analyses. This group shows very interesting features (e.g. behavioral isolation based on female mating choice, mutual sexual selection, partial hybrid viability and fertility, female heterogamety) which will allow us to investigate the issues discussed above. Our first task will focus on behavioral isolation and sexual selection. In this task we propose to estimate the relative roles of male and female mate choice in behavioral isolation, and ask how these roles are linked with sexual selection processes within species. In a second task we will describe postzygotic isolation and investigate genome wide genetic patterns of incompatibilities. We will be particularly keen to investigate the genetic patterns associated with ZW sex chromosomes.
We expect this project to yield new insights into the mechanisms associated with behavioral isolation and genetic incompatibilities within a unique study system, a major step towards understanding the evolution of isolating barriers. Moreover, this work focusing on marine invertebrates with a direct development will be a welcome complement to empirical studies of speciation in the sea, which are mainly focusing on invertebrates with a pelagic larval phase and lacking a strong behavioral component to speciation.