Sara Hanson and Danielle Beekman to present on Thursday (21 April 2011) BSB 2-501 at noon
Sara Hansons talk
Title: Genome and Transcriptome Analysis of Sexual and Asexual Reproduction in Monogonont Rotifers
Abstract: How sexual reproduction has persisted in nature is an important and unsolved evolutionary question. As cyclical parthenogens, monogonont rotifers have overcome constraints on the loss of sexual reproduction in order to frequently transition between sexual and asexual generations. This makes monogononts a powerful system with which toaddress the maintenance of sex in animals. However, the molecular nature of meiosis and parthenogenesis in these species is poorly understood. To expand our knowledge of the molecular mechanisms of monogonont reproduction, we sequenced draft genomes of two distantly related species, Brachionus calyciflorus and B. manjavacas. We searched the genome of each species for genes involved in meiotic processes, and identified over eighty meiotic gene homologs, several of which have undergone duplication events specific to the monogonont lineage. In addition, global gene expression patterns were determined through generation of mRNA-seq libraries from obligate parthenogenetic (OP) and cyclical parthenogenetic (CP) strains of B. calyciflorus. Quantitative comparison of expression between these libraries revealed several differentially expressed genes specific to sexual and asexual reproduction in this species. The presence of gene duplications and differential expression between OP and CP strains is consistent with data from cyclical parthenogenetic arthropod species, suggesting mechanisms for convergent evolution of this reproductive mode. Furthermore, establishing molecular markers for sex and asex in monogononts will allow for more informed analyses of our ongoing gene expression studies in ancient asexual bdelloid rotifers, who possess a number of genes specific to meiosis.
Danielle Beekmans Talk
Title: Establishing The Origin And Molecular Evolution Of Dipteran Male Achiasmatic Meiosis
Abstract: Drosophila melanogaster is unique amongst model organisms in that males undergo an unusual form of meiosis in which synaptonemal complex (SC) formation and recombination do not occur. This type of meiosis is called achiasmatic meiosis and is exceptional: nearly all eukaryotes, includingDrosophila females, require SC formation and recombination for successful completion of meiosis. A number of genes have been identified that are specific to Drosophila male achiasmatic meiosis and many of these genes are paralogous to non-meiotic genes. Our work investigates the origin and evolution of male meiosis-specific genes in a number of ways. We searched 27 dipteran species outside of Drosophila for two sets of paralogous genes: SA/snm andborr/aust. The male meiosis-specific paralogs (snm and aust) were not found in these species, thus these genes appear to be Drosophila-specific. However, in three close relatives of Drosophila, a second SA variant was found. This indicates that the SA duplication event, and potentially other duplications leading to the origin of an achiasmatic meiosis gene, may have occurred prior to the divergence of Drosophila. We have also analyzed the evolutionary rates of many male achiasmatic meiosis genes within 12 Drosophila species. These genes are evolving at a very rapid pace compared to their paralogs, and this trend is seen in all Drosophila species examined. This rapid evolution suggests that positive selection is acting upon these genes, adjusting them for their roles in achiasmatic meiosis. Changes in gene expression and splicing have played a part in the evolution of male achiasmatic meiosis as well. Surprisingly, a number of male meiosis-specific genes are expressed in females and vice versa. Preliminary data from several Drosophila species suggests that alternative splicing is playing a role in regulating the products of these transcripts in the different sexes. This study is currently being expanded to include other dipteran taxa. By excluding SC formation and recombination from the meiotic process, yet still producing viable gametes, Drosophila males are essentially cheating their way through meiosis. Investigating the origin and evolution of Drosophilamale achiasmatic meiosis from a variety of different angles allows us to better understand this process and provides additional insight on the origin, evolution and function of meiosis.