Concerted evolution of a tandemly arrayed family of mating-specific genes in Phytophthora analyzed through inter- and intraspecific comparisons

Abstract

Multigene families are features of most eukaryotic genomes, which evolve through a variety of mechanisms. This study describes the structure, expression, and evolution of a novel family in the oomycete Phytophthora. In the heterothallic species P. infestans, M96 is expressed specifically during sexual sporogenesis, and encodes a low-complexity extracellular protein that may be a component of oospore walls. Intriguingly, M96 exists in P. infestans as 22 relatively homogeneous loci tandemly repeated at a single site, which is partitioned by inversions and retroelements into subclusters exhibiting semi-independent evolution. M96 relatives were detected in other heterothallic and homothallic oomycetes including species closely (P. mirabilis, P. phaseoli) or distantly (P. ramorum, P. sojae) related to P. infestans. Those M96 relatives also exhibit oosporogenesis-specific expression and are arrayed multigene families. Nucleotide changes and repeat expansion diversify M96 in each species, however, paralogues are more related than orthologues. Concerted evolution through gene conversion and not strong purifying selection appears to be the major contributor to intraspecific homogenization. Divergence and concerted evolution was also detected between isolates of P. infestans. The divergence of M96 proteins between P. infestans, P. ramorum, and P. sojae exceeds that of typical proteins, reflecting trends in reproductive proteins from other kingdoms.