Courtesy of William Snell, University of Texas Southwestern, USA.

Although it is known that fertilization triggers events that ultimately lead to the transcriptional activation of the zygotic genome, we know surprisingly little about these events — a factor that probably contributes to the low success rates of current mammalian-cloning efforts. So, to investigate the mechanisms that control zygotic gene transcription, Zhao et al. turned to Chlamydomonas reinhardtii — a unicellular alga — in which early zygotic development is easier to study. Here they report that the gamete-specific, homeodomain protein GSP1 can activate the transcription of some zygotic genes in the absence of gamete fusion. Their findings show that both gametes contribute proteins that are required for zygote development.

Chlamydomonas's quirky life cycle makes it amenable to studies of fertilization. Upon nutrient starvation, its haploid vegetative cells of two mating types — mt− and mt+ — undergo gametogenesis. When gametes of opposite mating type meet, they fuse together (see picture), triggering rapid zygote-specific gene expression. This expression occurs independently of protein synthesis, indicating that pre-exisiting factors in one or both gametes regulate the expression of these genes. As zygote maturation continues, zygotes clump together to form large aggregates and mt−-derived choloroplasts are selectively degraded — zygotes germinate on return to a nutrient-rich medium.

The previous finding that GSP1 is present only in mt+ gametes led the authors to consider it as a candidate regulator of zygotic gene transcription in Chlamydomonas. To test this, they expressed gsp1 in mt− cells, and found that although the vegetative mt− transformants appeared normal, they formed zygote-like aggregates on undergoing gametogenesis — a behaviour not seen in transformed mt+ cells. The transformed mt− gametes expressed six out of seven zygote-specific genes — the unexpressed gene, ezy1, is thought to be required for mt− chloroplast destruction. Its lack of expression in mt− transformants indicates that it is under separate regulatory control or perhaps is imprinted in mt− gametes.

But if GSP1 regulates zygote-specific genes in mt− cells, why does it not affect zygotic gene expression in mt+ gametes? Perhaps this is because, as the authors suggest, zygote-specific gene promoters are inaccessible to GSP1 in mt+ gametes or because GSP1 associates with a pre-existing mt–-specific partner molecule to form a transcription-regulatory complex that activates zygote-specific gene expression. The parallels between these events and those in budding yeast indicate that atypical, gamete-specific, homeodomain proteins, such as GSP1 and budding yeast's MATα2, might have evolved to act as regulators of zygotic gene expression before animals and plants diverged. If so, then such studies could inform our understanding of how zygote gene expression is controlled in mammals.