Sex chromosome evolution: historical insights and future perspectives.

Many separate-sexed organisms have sex chromosomes controlling sex determination. Sex chromosomes often have reduced recombination, specialized (frequently sex-specific) gene content, dosage compensation and heteromorphic size. Research on sex determination and sex chromosome evolution has increased over the past decade and is today a very active field. However, some areas within the field have not received as much attention as others. We therefore believe that a historic overview of key findings and empirical discoveries will put current thinking into context and help us better understand where to go next. Here, we present a timeline of important conceptual and analytical models, as well as empirical studies that have advanced the field and changed our understanding of the evolution of sex chromosomes. Finally, we highlight gaps in our knowledge so far and propose some specific areas within the field that we recommend a greater focus on in the future, including the role of ecology in sex chromosome evolution and new multilocus models of sex chromosome divergence.

Sex-limited experimental evolution drives transcriptomic divergence in a hermaphrodite.

The evolution of gonochorism from hermaphroditism is linked with the formation of sex chromosomes, as well as the evolution of sex-biased and sex-specific gene expression to allow both sexes to reach their fitness optimum. There is evidence that sexual selection drives the evolution of male-biased gene expression in particular. However, previous research in this area in animals comes from either theoretical models or comparative studies of already old sex chromosomes. We therefore investigated changes in gene expression under 3 different selection regimes for the simultaneous hermaphrodite Macrostomum lignano subjected to sex-limited experimental evolution (i.e. selection for fitness via eggs, sperm, or a control regime allowing both). After 21 and 22 generations of selection for male-specific or female-specific fitness, we characterized changes in whole-organism gene expression. We found that female-selected lines had changed the most in their gene expression. Although annotation for this species is limited, gene ontology term and Kyoto Encyclopedia of Genes and Genomes pathway analyses suggest that metabolic changes (e.g. biosynthesis of amino acids and carbon metabolism) are an important adaptive component. As predicted, we found that the expression of genes previously identified as testis-biased candidates tended to be downregulated in the female-selected lines. We did not find any significant expression differences for previously identified candidates of other sex-specific organs, but this may simply reflect that few transcripts have been characterized in this way. In conclusion, our experiment suggests that changes in testis-biased gene expression are important in the early evolution of sex chromosomes and gonochorism.

Rapid evolution of sex role specialization in a hermaphrodite under sex-limited selection.

The evolution of separate sexes from hermaphroditism is thought to have occurred independently many times, and may be linked to the evolution of sex chromosomes. Even though we have a good understanding of the theoretical steps in the evolution of sex chromosomes from a hermaphrodite ancestor, the initial stages are still hard to study in animals because many well-studied animal sex chromosome systems are old. We addressed this problem by experimentally selecting a hermaphrodite via sex-limited experimental evolution for several generations, simulating the early stages in the evolution of a sex chromosome. After 14 generations, a fitness assay revealed evidence of incipient sex role specialization in the female-selected lines, presumably reflecting the release from constraints usually imposed by selection on the other sex role. Importantly, however, this was not simply explained by morphology because testis and ovary sizes did not diverge among treatments. There was no evidence of a change in the male-selected lines. Our study shows that sex role specialization can occur rapidly as a result of sex-limited selection, which is consistent with genetic constraints between sex roles, and in line with the first predicted steps toward the evolution of a new sex chromosome system.

Evolutionary biologists have developed detailed theories which attempt to explain the evolution of sex chromosomes and separate sexes. Unfortunately, testing these theories can be challenging, since most of the best-studied sex chromosome systems are many millions of years old. This makes it difficult to disentangle cause and effect during sex chromosome evolution. In this study, we have tried to re-create the origin of sex chromosome and separate sexes from a hermaphroditic ancestor within the laboratory. Our aim was to better understand early sex chromosome evolution in real time. For this, we carried out experimental evolution in the simultaneously hermaphroditic flatworm Macrostomum lignano. When it mates, this species both receives sperm from the partner, and donates sperm back. We developed a genetic marker-based selection protocol which allowed us to restrict the worms' reproduction, so that the male-selected lines could only produce offspring through sperm, and the female-selected lines could only produce offspring through eggs. After 14 generations of selection, we found that individuals from the female-selected lines became better at laying eggs, but worse at fertilizing their partners. However, the difference did not seem to be explained by changes in gonad size, since there were no differences between male- and female-selected worms in testes or ovary size. These results show that sexual specialization may be possible to evolve on surprisingly short time scales.