Rapid Evolution of a Pollen-specific Oleosin-like Gene Family from Arabidopsis Thaliana and Closely Related Species

It has been shown in a variety of species that genes expressed in reproductive tissues evolve rapidly, which often appears to be the result of positive Darwinian selection. We investigated the evolution of a family of seven pollen-specific oleosin-like proteins (or oleopollenins) in Arabidopsis thaliana and two closely related species. More than 30 kb of a genomic region that harbors the complete, tandemly repeated oleopollenin cluster were sequenced from Arabidopsis lyrata ssp. lyrata, and Boechera drummondii. A phylogenetic analysis of the complete gene cluster from these three species and from Brassica oleracea confirmed its rapid evolution resulting from gene duplication and gene loss events, numerous amino acid substitutions, and insertions/deletions in the coding sequence. Independent duplications were inferred in the lineages leading to Arabidopsis and to Brassica, and gene loss was inferred in the lineage leading to B. drummondii. Comparisons of the ratio of nonsynonymous (d(N)) and synonymous (d(S)) divergence revealed that the oleopollenins are among the most rapidly evolving proteins currently known from Arabidopsis and that they may evolve under positive Darwinian selection. Reverse transcriptase polymerase chain reaction analysis demonstrated the expression of oleopollenins in flowers of the outcrossing A. lyrata, the selfing B. drummondii, and the apomictic Boechera holboellii, suggesting that oleopollenins play an important role in species with different breeding systems. These results are consistent with a putative function in species recognition, but further analyses of protein function and sequence variation in species with different breeding systems are necessary to reveal the underlying causes for the rapid evolution of oleopollenins.