Transcriptome analysis of sexual and vegetative development in Volvox carteri

Genome-enabled approaches to understanding life cycle transitions in Volvox have great promise for elucidating regulatory networks that control growth and development. Deep transcriptome sequencing (RNA-seq) allows comprehensive interrogation and quantitation of gene expression patterns and is an ideal tool to use for identifying gene regulatory networks (GRNs). We performed RNA-seq analyses from samples of synchronous Volvox carteri cultures from each sex at two or three hour intervals across all stages of sexual and vegetative development to generate 64 transcriptomes. We have begun to mine these data by identifying genes that are expressed specifically during the sexual cycle of males and females. We identified three classes of sex-regulated genes that were expressed only during sexual development: male specific, female specific and non-gender specific. Among each group were genes expressed early in development as well as those that were expressed in terminally differentiated germ cells. Interestingly, male specific genes were the most numerous class among the three supporting the idea that spermatogenesis entails a more complex developmental and cell-type specialization program than oogenesis. The male specific genes included cell cycle/cell division/embryogenesis-related ones such as cyclin D, DP1, inversion kinesin InvA and regA-like protein RlsC to produce small and abundant sperm cells in sperm packets. In those groups, we also recognized homologs of several known volvocine sex-related genes such as GSP1, GSM1 and GCS1/HAP2.