This paper is the first in a series of papers that is emerging from Banoo Malik's PhD thesis and is a result of a long-standing project with former postdoc Marilee Ramesh (now at Roanoke College). It's also the first of a number of meiotic "gene stories" that we have been untangling over the past few years. The image shown is a summary of the phylogenetic distribution of Spo11 homologs that we determined. The paper is not Open Access (sorry); however, if you are interested in reading it and do not have a subscription to MB&E, drop me an email.
Spo11 is a meiotic protein of fundamental importance as it is a conserved meiosis-specific transesterase required for meiotic recombination initiation in fungi, animals and plants. Spo11 is homologous to the archaebacterial topoisomerase VIA (Top6A) gene, and its homologs are broadly distributed among eukaryotes, with some eukaryotes having more than one homolog. However, the evolutionary relationships among these genes are unclear, with some debate as to whether eukaryotic homologs originated by lateral gene transfer. We have identified and characterized protist Spo11 homologs by degenerate PCR and sequencing and by analyses of sequences from public databases. Our phylogenetic analyses show that Spo11 homologs evolved by two ancient eukaryotic gene duplication events prior to the last common ancestor of extant eukaryotes, resulting in three eukaryotic paralogs: Spo11-1, Spo11-2 and Spo11-3. Spo11-1 orthologs encode meiosis-specific proteins and are distributed broadly among eukaryotic lineages, though Spo11-1 is absent from some protists. This absence coincides with the presence of Spo11-2 orthologs, which are meiosis-specific in Arabidopsis and are found in plants, red algae and some protists, but absent in animals and fungi. Spo11-3 encodes a Top6A subunit that interacts with topoisomerase VIB (Top6B) subunits, which together play a role in vegetative growth in Arabidopsis. We identified Spo11-3 (Top6A) and Top6B homologs in plants, red algae, and a few protists, establishing a broader distribution of these genes among eukaryotes, indicating their likely vertical descent followed by lineage-specific loss.