At the journal club last week (13 March), we discussed the amazing study about the genetic basis of Batesian mimicry in Papilio polytes by Kunte et al. (2014, Nature). A supergene complex was expected, a single genes was detected - doublesex. Mimicry controlled by gene expression regulation (but how?), but not by specific spliced iso-forms. A really nice paper and a must read for every evolutionary geneticist.
At the Journal Club 6 March 2014 we discussed a recent paper by Balakrishnan et al. (2013, Open Biology) where the transcriptome of the violet-eared waxbill is presented and analysed in relation to zebra finch. Details of which transcripts are present in the brain of this species are given, but the most interesting parts relate to diversity and evolutionary rate (dN/dS) analyses. As expected the Z chromosome was an outlier and showed little genetic variation and higher rate of evolution. However, unexpectedly one other chromosome, chr4a, was also an outlier but in the oposite direction: high diversity and low dN/dS. Why? Still not known, but we note that this is also the chromosome that has formed a neo-sex chromosome in birds (Pala et al. 2012, Heredity).
In a recent paper published this week in PLoS One, we evaluate the degree of population genetic differentiation and fixation of the Canary Islands blue tit subspecies complex using microsatellite markers and aim to get insights in the population history using coalescence based methods (Hansson et al. 2014).
The Canary Island populations were strongly genetically differentiated and had reduced diversity with pronounced fixation including many private alleles. In population structure models, the relationship between the central island populations (La Gomera, Tenerife and Gran Canaria) and El Hierro was difficult to disentangle whereas the two European populations showed consistent clustering, the two eastern islands (Fuerteventura and Lanzarote) and Morocco weak clustering, and La Palma a consistent unique lineage. Coalescence based models suggested that the European mainland forms an outgroup to the Afrocanarian population, a split between the western island group (La Palma and El Hierro) and the central island group, and recent splits between the three central islands, and between the two eastern islands and Morocco, respectively.
It is clear that strong genetic drift and low level of concurrent gene flow among populations have shaped complex allelic patterns of fixation and skewed frequencies over the archipelago. However, understanding the population history remains challenging; in particular, the pattern of extreme divergence with low genetic diversity and yet unique genetic material in the Canary Island system requires an explanation. A potential scenario is population contractions of a historically large and genetically variable Afrocanarian population, with vicariance and drift following in the wake. The suggestion from sequence-based analyses of a Pleistocene extinction of a substantial part of North Africa and a Pleistocene/Holocene eastward re-colonisation of western North Africa from the Canaries remains possible. We are currently working on additional studies to resolve the colonisation scenario based on a large set of introns and RAD-tag sequencing.