Evolution of Spore Release Mechanisms in the Saprolegniaceae (Oomycetes): Evidence from a Phylogenetic Analysis of Internal Transcribed Spacer Sequences

Abstract

Classical studies on spore release within the Saprolegniaceae (Oomycetes) led to the proposition that different mechanisms of sporangial emptying represent steps in an evolutionary transition series. We have reevaluated this idea in a phylogenetic framework using internal transcribed spacer sequences of four genera. These data were compared with the response to osmotic stress exhibited by each taxon.Saprolegniaemerges as the most basal genus, sister toAchlya, Thraustotheca,andDictyuchus. AchlyaandThraustothecaare most closely related, whileDictyuchusappears to have evolved along a separate evolutionary lineage. The resulting phylogenetic framework is consistent with the idea that the mechanism of sporangial emptying exhibited bySaprolegniarepresents the plesiomorphic condition from which the other mechanisms were derived independently. These alternative mechanisms of spore release may have resulted from a small number of mutations that inhibited axonemal development and altered the temporal and spatial expression of lytic enzymes that degrade the sporangial wall.