Abstracts (first author)


A young neo-sex chromosome is the site of divergence between incipient stickleback species

Author(s): Kitano J, Yoshida K


Sex chromosomes turn over rapidly in some taxonomic groups, where closely related species have different sex chromosomes. However, little is known about the functional roles of sex chromosome turnover in phenotypic diversification and speciation. We use a sympatric pair of threespine sticklebacks to understand the roles of sex chromosome turnover in phenotypic divergence and speciation. Previously, we found that the Japan Sea sticklebacks have a neo-sex chromosome system resulting from a fusion between an ancestral Y chromosome and an autosome, while sympatric Pacific Ocean sticklebacks have a XY sex chromosome system. Furthermore, we demonstrated that the Japan Sea neo-X chromosome plays an important role in phenotypic divergence and reproductive isolation between these sympatric species. Here, we conducted additional QTL mapping of other morphological and behavioral traits to confirm that both old-X and neo-X chromosomes have significant QTL important for morphological divergence, while the old-X, but not the neo-X chromosome, contributed to hybrid courtship abnormality. Next, we conducted whole genome sequencing of these two sticklebacks to find that the Japan Sea neo-Y chromosome has no large-scale deletion, but some regions on the neo-Y have started to accumulate deleterious mutations. Importantly, significant QTL on the neo-X chromosome were located at regions even outside the regions where deleterious mutations accumulate. Furthermore, comparison of genome sequences between these two species revealed that the neo-X chromosome showed faster protein evolution than autosomal genes. Thus, a young neo-sex chromosome is the site of divergence between incipient species.

Abstracts (coauthor)

Testing the link between heterochromatin evolution and speciation in sticklebacks

Author(s): Yoshida, K, Makino T, Yamaguchi K, Shigenobu S, Hasebe M, Mori S, Peichel K, Kawata M, Toyoda A, Fujiyama A, Kitano J


Genes that cause hybrid incompatibility between closely related species have not yet been identified in natural vertebrates. Stickleback fishes are a great model for identifying speciation genes, because there are multiple sympatric species pairs (i.e., pairs of incipient species that are sympatric, but are reproductively isolated). Our previous studies identified several QTLs of isolating barriers, including hybrid male sterility, between two sympatric incipient species of Japanese threespine stickleback fishes. Theoretical study suggests that heterochromatin-binding proteins can rapidly evolve in selfish manner and may affect the genetic incompatibility. Indeed, causal genes of hybrid male sterility in Drosophila and mice are rapidly evolving genes involved in heterochromatin regulation. To test the link between heterochromatin evolution and speciation in natural vertebrate systems, we determined the whole genome sequences of these two stickleback species and analyzed the sequence differences in the QTL regions in order to identify candidate speciation genes. Within the QTL regions of hybrid male sterility, we found two chromatin-binding domain containing genes with high Ka/Ks rate. Currently, we are analyzing the function of these genes in vitro using protein-protein interaction assays and in vivo using the transgenic sticklebacks.


Phenotypic plasticity plays important roles in adaptation to changing environments. However, organisms tend to lose the capacity for phenotypic plasticity under stable environments. Although variation in phenotypic plasticity has been found throughout the animal kingdom, molecular and genetic mechanisms underlying such variation remain elusive. We are addressing this question by using threespine stickleback (Gasterosteus aculeatus), because they have a variety of ecotypes that show different levels of phenotypic plasticity. After the last glacial recession, ancestral marine sticklebacks colonized newly formed freshwater habitats, resulting in extensive phenotypic diversification. In this study, we report that whole transcriptome analysis revealed that marine sticklebacks exhibited significant photoperiodic response of expression levels of thyroid hormone-stimulating hormone beta 2 (TSHß2) gene, whereas such response was lost in freshwater sticklebacks. Loss of TSHß2 response independently occurred in both North American and Japanese freshwater populations. Further analyses of TSHß2 response in F1 hybrids demonstrated that loss of TSHß2 response has different genetic basis between Japanese and North American populations. Genome sequencing further revealed several ecotype-specific SNPs at the cis-regulatory region of TSHß locus in North America freshwater populations. We are currently conducting luciferase-reporter assays in vitro to investigate the cis-regulatory mechanisms of TSHß2 response. Furthermore, we are making TALEN-induced knockout sticklebacks to understand the roles of TSHß2 in vivo. Thus, further studies on the variation in TSHß2 response will lead to a better understanding of the genetic mechanisms underlying convergent loss of phenotypic plasticity.


Larvae of the Hokkaido salamander (Hynobius retardatus) exhibit two distinct morphs, "attack morphs" and "defense morphs", as a result of inducible phenotypic response to preys and predators, respectively. The existence of preys, Rana pirica tadpoles, leads to the induction of attack morphs, which have broad heads likely suitable for catching tadpoles. The existence of predators, dragonfly larvae (Aeshna nigroflava), induces them to become defense morphs, which have enlarged external gills and high tails suitable for avoiding the predatory attacks. However, molecular mechanisms underlying this phenotypic plasticity have yet to be elucidated. To reveal the developmental and physiological mechanisms of this phenotypic plasticity, we carried out de novo transcriptome analysis of the Hokkaido salamanders. First, we collected eggs in the wild and then hatched them in laboratory. The larvae were exposed to either predators or preys to induce different morphs. Morph induction was completed about 7 days after the start of exposure. RNAs were extracted from 4 tissues (brain, head, external gill and tail) and 3 time points (0 hour, 12 hours, and 7 days after the exposure onset) and sequenced. Obtained reads and contigs of treatment samples were compared with those of control samples (i.e. no exposure) to identify differentially expressed genes. Approximately 2,000 genes were identified as differentially expressed genes at each time point and in each tissue. These genes include hormone related functional genes, such as COL2A1, HBA2, and NR2C1. RNA processing/splicing was found as an enriched functions in the Gene Ontology database, suggesting that the phenotypic plasticity of Hokkaido salamanders may be linked to regulation of alternative splicing.

The genetic architecture of the latitudinal variation in sexual dimorphism of medaka

Author(s): Kawajiri, M, Kohta Y, Fujimoto S, Mokodongan DF, Yamahira K, Kitano J


Sexual dimorphism, morphological differences between the sexes, is widespread a throughout the animal kingdom. However, the degree of sexual dimorphism varies considerably among closely-related species or even populations. Although there are many theoretical genetic models that can explain the genetic mechanisms of variation in sexual dimorphism, little empirical studies have been conducted to investigate the genetic basis of the variation in sexual dimorphism among natural populations. How many genes are involved in the variation in sexual dimorphism? Are they localized at particular chromosomal regions, such as sex chromosomes? Are different dimorphic traits regulated by different genes? In the Japanese medaka (Oryzias latipes), populations at lower latitude are more sexually dimorphic than populations at higher latitude. For example, anal and dorsal fins of mature males in low-latitude populations are longer than that of the higher-latitude males. Laboratory rearing experiments revealed that these differences in male fin length is mediated by the difference in the rate of fin elongation during development, in particular after a certain body size which probably represents sexual maturation. Furthermore, males from lower-latitude populations show more frequent fighting and courtship behaviors than males from higher-latitude populations. In this study, genetic basis for these morphological and behavioral differences were investigated using QTL mapping. We crossed two populations of medaka from habitats of different latitude (Aomori and Okinawa) and quantified several traits related to fin elongation process and mating behavior in F2 progenies. We next designed custom SNP assay system to create a linkage map and conduct QTL mapping. We found significant QTL controlling different sexually dimorphic traits on a single autosome, suggesting that these secondary sexual characters may be controlled by the same genetic mechanism.


Chairman: Octávio S. Paulo
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XIV Congress of the European Society for Evolutionary Biology

Organization Team
Department of Animal Biology (DBA)
Faculty of Sciences of the University of Lisbon
P-1749-016 Lisbon


Computational Biology & Population Genomics Group