Sex or no sex? Group I introns and independent marker genes reveal the existence of three sexual but reproductively isolated biospecies in Trichia varia (Myxomycetes)

Abstract

Plasmodial slime molds are members of the class Amoebozoa forming elaborate fruit bodies releasing airborne spores. Two species concepts have been developed independently: a morphological relying on fruit body characters, and a biological relying on crossing studies of a few cultivable species. In an attempt to reconcile both concepts, we obtained for 198 specimens of the common species Trichia varia partial sequences of three independent markers (nuclear small-subunit (SSU) ribosomal RNA gene, extrachromosomal; elongation factor 1 alpha gene, chromosomal; cytochrome oxidase subunit 1 gene, mitochondrial). The resulting phylogeny revealed 21 three-marker genotypes clustering into three groups. Combinations of the single-marker genotypes occurred exclusively within these groups, called 1, 2a, and 2b. To examine the suitability of group I introns to monitor speciation events, complete SSU sequences were generated for 66 specimens, which revealed six positions that can carry group I introns. For each of the groups 1 and 2a, five of these positions were occupied by different intron genotypes; and no genotype was shared by the two groups. Group 2b was devoid of introns. Putatively functional or degenerated homing endonuclease genes were found at different positions in groups 1 and 2a. All observations (genotypic combinations of the three markers, signs of recombination, intron patterns) fit well into a pattern of three cryptic biological species that reproduce predominantly sexual but are reproductively isolated. The pattern of group I introns and inserted homing endonuclease genes mounts evidence that the Goddard-Burt intron life cycle model applies to naturally occurring myxomycete populations.