Gene and protein structure of the mate recognition protein gene family in Brachionus manjavacas (Rotifera)

The mate recognition protein (MRP) gene is a member of a family of extracellular matrix protein genes, called MRP Motif Repeat (MMR) genes, with no known homologs. Two sets of MMR genes, designated MMR-A and MMR-B, were found in Brachionus manjavacas. MMR-B has previously been shown to encode the MRP in the Brachionus plicatilis species complex. MMR family genes share the same basic structure: a signal peptide sequence, followed by nearly identical 276 bp (MMR-A) or 261 bp (MMR-B) repeats, with a truncated final repeat. Each repeat of the predicted MMR-A and -B proteins is expected to have a secondary structure of 5 alpha-helices, ranging in length from 11 to 20 amino acids, separated by coils of 1-3 amino acids. Hydrophobic and hydrophilic amino acids are predicted to be partitioned to opposite sides of each alpha-helix, suggesting that MMR proteins are globular with a hydrophobic core. MMR-A and MMR-B proteins vary in their post-translational modifications, resulting in differences in size and charge, and likely causing differences in the physical properties of the proteins on the surface of the female, and their ability to be recognized by a receptor on a male rotifer. The identity of MMR gene repeats is theorized to be maintained by concerted evolution, through a process of unequal crossing over and/or gene conversion, with new mutations likely to be lost. Rarely, however, the same process of concerted evolution can rapidly spread a mutation across all of the repeats. When a mutation results in conformational changes in the protein detectable by males, it could lead to reproductive isolation and thereby to speciation. Thus, changes in MRP could be a driving force in the high degree of species diversity seen within the B. plicatilis cryptic species complex.