Species relative abundance and direction of introgression in oaks

Abstract

Successful hybridisation and subsequent introgression lead to the transfer of genetic material across species boundaries. In this process, species relative abundance can play a significant role. If one species is less abundant than the other, its females will receive many heterospecific gametes, increasing mate-recognition errors and thus hybridisation rate. Moreover, first-generation hybrids will also more likely mate with the more abundant species, leading to asymmetric introgression. These predictions have important fundamental consequences, especially during biological invasions or when a rare species threatened by extinction is surrounded by individuals from a related species. However, experimental tests in nature of the importance of the relative abundance of each species on hybridisation dynamics remain scarce. We assess here the impact of species relative abundance on hybridisation dynamics among four species from the European white oak species complex. A total of 2107 oak trees were genotyped at 10 microsatellite markers and Bayesian clustering methods were used to identify reference trees of each species. We then used these reference trees to simulate purebred and hybrid genotypes to determine optimal threshold for genetic assignment. With this approach, we found widespread evidence of hybridisation between all studied oak species, with high occurrence of hybrids, varying from 11% to 31% according to stand and sampling strategies. This finding suggests that hybridisation is a common phenomenon that plays a significant role in evolution of this oak species complex. In addition, we demonstrate a strong impact of species abundance on both hybridisation rate and introgression directionality.

Human-induced hybridization among congeneric endemic plants on Tenerife, Canary Islands

“…These populations contained not only putative F1 individuals that showed approximately 50 % admixture between two species, but also backcrossed individuals and even some individuals that showed admixture between all three parental species. Such complex patterns of hybridization are usually interpreted as evidence that hybridization has been going on for several generations (Lepais et al 2009;Thompson et al 2010).…”

Genetic evidence of reproductive isolation in a remote enclave of Quercus pubescens in the presence of cross-fertile species

“…In general, sharp genetic delineation between sympatric species is not very new for the European white oak complex. Several authors have recently found strong genetic separation of species in mixed stands across the main range of coexistence of different oak species (Gugerli et al 2007;Curtu et al 2007;Lepais et al 2009). However, our results provide new evidence for strong reproductive barriers and/or selection mechanisms among European oak species that shape their ecology and evolution at the Northern range margin.…”

“…The earlier surveys of leaf morphology suggested that trees in the study population reveal great variation spanning among three species: Q. pubescens, Q. petraea and Q. robur (Celiński and Filipek 1958;Staszkiewicz 1977;Danielewicz et al 2002). However, because these species are potentially cross-fertile (Lepais et al 2009;Salvini et al 2009;Lepais and Gerber 2011), we assumed that the study population embraces all 2,200 individuals, which however can mate non-randomly due to taxonomic or other barriers (e.g. distance, flowering synchrony).…”

“…To strengthen the power of multilocus clustering analysis (see below), we included the loci that appeared to be highly discriminative in the earlier studies on European white oaks (e.g. Curtu et al 2007;Gugerli et al 2007;Lepais et al 2009). PCR products were sized using an ABI 3130XL sequencer (Applied Biosystems, Foster City, CA, USA) and the attached software (Genemapper 4.0).…”

“…Given that the study stand is a mixture of three oak species, it becomes evident that the population of Q. pubescens in Bielinek is being exposed to inter-specific pollen and subjected to a strong pressure of introgression, which seems to be stronger than within previously studied populations of the species complex (e.g. Finkeldey 2001;Curtu et al 2007Curtu et al , 2009Salvini et al 2009;Lepais et al 2009;Lepais and Gerber 2011). Although the introgression from Q. petraea could be the case, accompanying or alternative processes such as genetic drift and/or selection at the extremes of species ecological spectrum cannot be definitely excluded.…”