Identifying the mechanisms of speciation is one of the central pursuits in evolutionary biology. Evidence is mounting that prezygotic reproductive isolation may occur more quickly than post-zygotic isolation, effectively preventing sympatric populations from interbreeding
[1–5]. Pheromone-based mate recognition has become a focus in the study of prezygotic isolation due to the apparent high specificity in signaling, the vast diversity of signaling systems between species, its independence from environmental differences, and the direct effect of chemical cues in preventing matings between divergent types
The evolution of chemically mediated prezygotic barriers to reproduction may play an important role in speciation. As the signal for mate recognition diverges within or between populations, formerly compatible strains may become reproductively isolated, initiating the formation of new species. While examples of compounds involved in mate choice are known in organisms ranging from yeast to humans, and the resulting mating behavior is well described, the genetic bases and evolution of these chemical cues are thoroughly characterized for relatively few species (e.g.
[8–10]). Often, the correlation between cue diversity, evolution, and mating cannot be made directly due to the difficulty in characterizing the mating cue, identifying the gene(s) giving rise to the (often extremely complex) chemical signal, and conducting mate recognition studies in the same populations.
In this study, we employed molecular genetics, phylogenetics, and behavioral assays to examine the evolution and role of mate choice within the Brachionus plicatilis group, a cryptic species complex of monogonont rotifers. Monogonont rotifers are cyclically parthenogenetic; they generally reproduce asexually, but through a quorum sensing process or due to environmental factors such as temperature, food conditions, or pH, will produce males and undergo sexual reproduction
[11–14]. Members of the B. plicatilis group are found in inland saline environments around the world, often as sympatric species. Molecular phylogenetics based on coxI and its1 suggest the group is composed of 13 – 24 morphologically inconclusive phylotypes
[15–18], although only four species have been formally described: B. rotundiformis tschugunoff 1921; B. plicatilis of müller 1786, B. ibericus ciros-perez 2001, and B. manjavacas fontaneto 2007
[19–21]. The species complex is split into two main clades, “A” and “B,” with B. rotundiformis in clade “C,” and additional phylotypes falling into additional groups
[16, 17]. Crosses between different phylotypes have shown a gradient in mate recognition and copulation between members of the species complex
While much of the empirical and theoretical literature about mate choice focuses on female selection of male traits, males appear to play the predominant role in selecting a mate among B. plicatilis populations. A male rotifer randomly encounters a female, circles the female closely, localizes on her corona, and—if the female is recognized as compatible—copulates by hypodermic insemination
. Recognition is mediated by the Mate Recognition Protein (MRP), a glycoprotein on the surface of females
[24, 25]. Removal of MRP by EDTA causes cessation of male circling, and re-application of MRP to the surface of conspecific females, to females of a reproductively isolated phylotype, or even to plastic beads, restores male circling and copulatory behavior
The gene encoding MRP is the eponymous member of the MRP Motif Repeat (MMR) gene family
[25, 27]. MMR genes share the same basic structure: a signal peptide sequence, followed by one to nine nearly identical 276 bp (mmr-a) or 261 bp (mmr-b) “full” repeats, with a terminal repeat of 243 bp in which the final 11 (in mmr-a) or 6 (in mmr-b) codons are replaced by 4 or 2 non-homologous codons. MMR-B genes were recently shown through RNAi knockdown to be responsible for mate recognition in B. manjavacas[25, 28].
Here we describe the diversity and evolution of mmr-b within 27 clonal isolates representing 11 phylotypes, or probable species, within the B. plicatilis species complex. Using results of assays of mating behavior between phylotypes, we correlated mate recognition with genetic distance to determine the relationship between mmr-b and prezygotic reproductive isolation. Insights into the genetic basis of mate recognition and the mode of evolution of mmr-b indicate that mate recognition, mediated by MMR-B, plays a driving role in speciation within the cryptic species complex.