This post describes the most recent discussed paper (March 13,2014 discussion) “Patterns of reproductive isolation in Mediterranean deceptive orchids” (Scopece et al. 2007, http://dx.doi.org/10.1111/j.1558-5646.2007.00231.x).
We linked this paper with the Speciation topic, because its goal was to compare the level of interbreeding capabilities between species that differ in the reproductive isolation mechanisms. This isolation is somehow an ultimate proof of the completion of speciation process. The authors performed experimental crosses in orchids (in sexually or food deceptive species, in figure) and quantify their hybrids viability. Sexual and food deceptive species basically differ in how they “cheat” on their pollinators, but the consequence of this, is a difference on how their gene flow barriers are built up.
In a nutshell, their results highlight that pre-mating mechanisms (differential use of pollinators in sexually deceptive species) resulted in a larger number of successful crosses (fruit production) when artificially mated. This pre-mating isolation seems to be a “quick”, less costly, but weak way to build up reproductive isolation. In contrast, crosses between food deceptive species were less successful (large reproductive isolation), rather due to pre-mating causes as they may share pollinators, but to post-mating incompatibilities. From scarce genetic comparisons the authors showed larger genetic distances between food deceptive species, suggesting that the accumulation of genetic incompatibilities in the post-mating isolation case could be longer process.
We commented, also discussed by Sobel & Randle (2009, http://dx.doi.org/10.1111/j.1558-5646.2009.00691.x), some issues in the paper that may bias the comparison. These include the way they examined the pollinator associations, and the differences in species age for each group. However, Scopece et al. (2009, http://dx.doi.org/10.1111/j.1558-5646.2009.00693.x) nicely explained how decisions were taken in order to minimize their impact in the results.
Scopece et al.’s paper indicated how biological differences between species may impact the process of reproductive isolation in few orchid genera. Intriguing results, as the very variable interspecific distance between sexual and food deceptive orchid pairs, suggest that the rate of species diversification or genetic divergence in the two groups could drastically differ. We’ll go back to these two issues in future posts.