The mate recognition protein gene mediates
reproductive isolation and speciation in the
Brachionus plicatilis cryptic species complex
Chemically mediated prezygotic barriers to reproduction likely play an important role inspeciation. In facultatively sexual monogonont rotifers from the Brachionus plicatilis crypticspecies complex, mate recognition of females by males is mediated by the Mate RecognitionProtein (MRP), a globular glycoprotein on the surface of females, encoded by the mmr-bgene family.
In this study, we sequenced mmr-b copies from 27 isolates representing 11phylotypes of the B. plicatilis species complex, examined the mode of evolution and selectionof mmr-b, and determined the relationship between mmr-b genetic distance and materecognition among isolates.
Results:
Isolates of the B.
plicatilis species complex have 1-4 copies of mmr-b, each composed of 2-9nearly identical tandem repeats. The repeats within a gene copy are generally more similarthan are gene copies among phylotypes, suggesting concerted evolution.
Compared tohousekeeping genes from the same isolates, mmr-b has accumulated only half as manysynonymous differences but twice as many non-synonymous differences. Most of the aminoacid differences between repeats appear to occur on the outer face of the protein, and theseoften result in changes in predicted patterns of phosphorylation.
However, we found noevidence of positive selection driving these differences. Isolates with the most divergentcopies were unable to mate with other isolates and rarely self-crossed.
Overall the degree ofmate recognition was significantly correlated with the genetic distance of mmr-b.
Conclusions:
Discrimination of compatible mates in the B. plicatilis species complex is determined byproteins encoded by closely related copies of a single gene, mmr-b.
While concertedevolution of the tandem repeats in mmr-b may function to maintain identity, it can also leadto the rapid spread of a mutation through all copies in the genome and thus to reproductiveisolation. The mmr-b gene is evolving rapidly, and novel alleles may be maintained andincrease in frequency via asexual reproduction.
Our analyses indicate that mate recognition,controlled by MMR-B, may drive reproductive isolation and allow saltational sympatricspeciation within the B. plicatilis cryptic species complex, and that this process may belargely neutral.
Author: Kristin E GribbleDavid B Mark Welch Credits/Source: BMC Evolutionary Biology 2012, 12:134
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