| Current phylogenies identify 8 major eukaryotic radiations; representatives within each radiation either engage in sexual cycles or else possess a highly conserved set of genes that are selectively expressed during meiosis. Hence the common ancestor to all modern eukaryotes apparently engaged in haploid-diploid transitions followed by meiotic reduction. Haploid-diploid transitions have for some time been known to be mediated by homeoprotein heterodimerization in the fungi. We have recently shown that homeoprotein heterodimerization involving Gsp1, contributed to the zygote by plus gametes, and Gsm1, contributed by minus gametes, is necessary and sufficient to drive the haploid-diploid transition in C. reinhardtii. Since fungi and algae last shared a common ancestor at least 1 BYA, we propose that homeoprotein heterodimerization featured in an ancient, and perhaps the pioneering, sexual strategy. Gsm1 is a TALE-class homeoprotein in the KNOX family found in all green organisms; Gsp1 is a TALE-class homeoprotein weakly homologous to the BELL family and designated BELL-related. All green algae examined to date carry BELL-related members, whereas land plants all carry true BELL members that heterodimerize with KNOX proteins to drive key developmental stages. All green algae also share a common life-cycle feature: the unicellular zygote forms a dormant diploid spore which then undergoes meiosis. By contrast, the zygote of land plants divides to form a multicellular sporophyte which includes cells capable of undergoing meiosis to generate haploid spores. We propose that the algal/land-plant divergence entailed the loss of the Gsm1/Gsp1 "idea" and its replacement with the KNOX/BELL "idea," allowing transitional organisms to escape the dormant-zygote "rut" of the algae and explore sporophytic opportunities.
|