Global transcriptome analysis of heterodimeric homeobox-driven zygote developmental program in Chlamydomonas reinhardtii

Chlamydomonas reinhardtii provides an important model to study sexual reproduction in a unicellular context. Its sexual cycle culminates in the formation of diploid zygotes which differentiate into dormant spores that eventually undergo meiosis. Mating between haploid plus and minus gametes induces rapid cell wall shedding via the enzyme g-Lysin; cell fusion is followed by by heterodimerization of sex-specific homeobox transcription factors, Gsm1 and Gsp1, and initiation of zygote-specific gene expression. Developing zygotes secrete novel cell-wall materials that create multicellular aggregates called pellicle. To investigate the genetic underpinnings of zygote differentiation, we performed comparative transcriptome analysis of both pre- and post-mating samples. We identified 253 genes specifically enriched in early zygotes, 92% of which are not expressed in gsp1 null zygotes, indicating that the Gsm1/Gsp1 heterodimer is the main activator of the early zygote program. A cohort of genes induced by g-Lysin is shown to be negatively regulated in early zygotes at the post-transcriptional level in a Gsm1/Gsp1-dependent manner, enabling prompt transition to zygotic wall assembly.