Hybridization and introgression in wild diploid potato species: role in evolution and implications on the species concept

MASUELLI RW; CAMADRO EL; ERAZZÚ LE; BEDOGNI MC; MARFIL CF

Marsella

Congreso; 13th Evolutionary Biology Meeting at Marseilles; 2009

Potatoes, Solanum L. section Petota Dumortier, are unique among crop plants and their close relatives in possessing a very broad range of biological diversity and a wide geographical distribution in the Americas. There are approximately 200 species, arranged into two subsections, Estolonifera Hawkes and Potatoe G. Don, according to whether they produce only stolons or both, stolons and tubers, respectively. Species in subsection Potatoe are arranged into two superseries and 19 series that include wild and cultivated forms, with 2n=2x=24 to 2n=6x=72 but with a predominance of diploids. These species are reproductively isolated by external and/or internal barrier that can hinder or prevent hybridization. The internal barriers can be either pre-zygotic or post-zygotic and, when incomplete, the opportunity is provided for intercrossing and the formation of fertile hybrids in areas of overlapping, since genome differentiation in the group is low and the functioning of n and 2n gametes provides the opportunity for gene flow among ploidy levels. Potatoes can reproduced both asexually and sexually (most diploids are obligate outbreeders due to the S-locus gametophytic self-incompatibility system, and are insect-pollinated as polyploids). Thus, their breeding system provides for success and opportunism both in maintaining superior genotypes in stable environments and in allowing rapid evolution of new forms under varying environmental conditions. Natural hybridization and subsequent gene flow within and between various ploidy levels often results in exceedingly complicated patterns of variation. In fact, potato species have been interpreted as a large pool of morphologically similar species, partially isolated by geographical and ecological barriers. Notwithstanding, the taxonomy of the group has been mostly based on morphological characters (only more recently, the use of molecular markers has been introduced) and most accessions in germplasm banks have been assigned specific ranks. In general, potato taxonomists have been reluctant to mention hybrid forms in nature, clearly underestimating their numbers and their possible role in introgression and speciation events, which are perceived as disturbances or challenges to the biological species concept and a breakdown of isolating mechanisms. Potato evolutionary studies have mainly focused in the origin of polyploids. But, based on the results of more than 25 years of research with native Argentinian species, we have strong evidences to hypothesize that recurrent hybridization, in particular interspecific hibridization, acts as a creative evolutionary force in diploid species by releasing variation in natural populations, because it can induce both genetic and epigenetic changes (the latter as the result of genomic stresses). The heritable variation released by hybridization can provide colonizing advantages to the fittest genotypes -that can be stabilized by clonal propagation- and, eventually, lead to homoploid hybrid speciation. We also propose that the species concept in the group needs to be revised at the light of our results, derived from the application of morphological, genetic and molecular approaches, and field observations of the earliest potato taxonomists.