homologs of well-known meiotic genes. The direct implications are that Giardia is, or was recently, capable of sexual reproduction and, thus, does not represent an ancient eukaryotic lineage that diverged before meiosis arose. Instead, the origin of meiosis predates the diver- plemental Data), all indicate that these genes are comOne possible interpretation of these observations is that plete, bona fide homologs of meiotic genes. diplomonads such as Giardia represent an "intermedi- Table 2 shows that the Giardia genome encodes hoate" (i.e., premeiotic) stage in eukaryotic evolution [35]. mologs of key genes required for meiosis. Homologs of We have evaluated this compelling hypothesis and can five meiosis-specific genes are present in Giardia: now reject it.Dmc1, Spo11, Mnd1, Hop1, and Hop2. Proteins encoded by these genes are not known to function outside of meiosis among AFP species. Of meiosis-specific Genes and Their Functions at the Core of Meiosis We used a phylogenomic approach [36] to identify and genes surveyed, Giardia does not appear to encode mutS homologs Msh4 and Msh5. Whether this is due to validate a core group of meiotic genes as indicators for the presence or absence of meiosis. Our selection of the loss of these genes or later evolutionary origins is unclear; the sparse distribution of Msh4/5 genes indimeiotic genes was based on previous studies in eukaryotic model systems-mostly AFP. We chose genes havcates some losses. Five meiosis-specific genes in Giardia in conjunction with other known meiotic genes proing major meiotic functions in AFP and conservation in sequence and function [2]. Gene products with similar vide strong evolutionary evidence for meiosis in the putatively asexual diplomonads. The presence of meiimportant roles in meiosis among known models have the greatest potential as meiotic indicators in diverse otic genes in Giardia and in the other protists surveyed (see below) extends the diversity of eukaryotes known to eukaryotes. Readily detectable protein sequence conservation among these genes %51ف( amino acid idencontain these genes. The collective presence of multiple genes that work together in meiotic recombination tity) was needed to identify potential homologs in evolutionarily distant species. We included some "meiosis strongly suggests that Giardia has or very recently had the capacity to undergo meiosis and, thus, sexual reprospecific" genes-null mutations in all of these genes (in S. cerevisiae, at least) are defective only in meiosis, and duction. The possibility that Giardia undergoes meiosis warrants further investigation; experiments will be remost genes do not function outside of meiosis, with some possible exceptions in mammals (Hop2 processes, present either from the loss of meiosis (despecific gene present in Giardia requires independent loss or gain of meiotic functions in all other eukaryotes rived asexuality) or prior to its invention (ancestral asexuality). As the numbers of meiotic genes mount, these known. The strong inference of meiosis being present...
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