4.3. Two species in sympatry
Species C. verrucosa sp. A and C. verrucosasp. B are distributed in sympatry along the Antarctic Peninsula. Both species are present in the Weddell Sea, Scotia Sea, Potter Cove, Shetland L45, Palmer Station, Paradise Bay and Rothera Stations (Fig. 3). The absence of C. verrucosa sp. B in some sampling stations may be explained by the low number of samples obtained on those sites. Because both species coexist in sympatry but maintain genetic differences among them, a reproductive barrier must be playing an important role in isolating the species. Broadcast spawners, likeC. verrucosa sensu lato , release gametes into the water column where fertilization occurs, thus the strength of the prezygotic reproductive barriers, like temporal isolation, can play a crucial role in reproductive isolation between species (Levitan et al., 2004). Another plausible hypothesis for reproductive isolation is gamete incompatibility, given that many studies on the evolution of gamete recognition proteins have shown that they tend to evolve more rapidly than other proteins, and frequently be under positive selection (Kosman & Levitan, 2014; Vacquier & Swanson, 2011). For example, in sea urchin species pairs, only 10 amino acid changes can lead to complete gamete incompatibility between species (Zigler, McCartney, Levitan, & Lessios, 2005). According to this, to perform experiments of reproduction betweenC. verrucosa sp. A and sp. B , would be helpful to understand the evolution and potential patterns of gene flow between these two closely related species.
The population structure of these species showed a striking and unexpected pattern, especially since they are sympatric and there are not significant known differences in biological, ecological or reproductive traits that could explain different dispersal potential.C. verrucosa sp. A showed genetic structured population, while no genetic structure was registered among the populations ofC. verrucosa sp. B . Within C. verrucosa sp. A , IBD analysis showed no relation between genetic and geographic distance (Supplemental information, Fig. S2). C. verrucosa sensu lato presents a wide distribution range and with high registered abundances all around the Antarctic continent (Kott & Mather, 1969; Monniot et al., 2011; Tatián et al., 1998; Tatián & Lagger, 2010; Turon et al., 2016), thus the continuity and high abundance of populations could allow genetic connectivity, keeping an active gene flow over large distances. Therefore, and with no more current knowledge, it could be hypothesized that populations of C. verrucosa sp. B presents a more continuous distribution pattern along its distribution range allowing a higher gene flow, while C. verrucosa sp. A presents discrete populations with more restricted gene flow. Indeed, it has been suggested for ascidians that high mutation rates in both the nuclear and the mitochondrial genomes enable the accumulation of genetic diversity in relatively isolated populations (Delsuc, Brinkmann, Chourrout & Philippe, 2006; Reem, Douek, Katzir & Rinkevich, 2013). This, in turn, could be related to different capabilities for colonizing different substrate types, and/or to differential dispersal potential among the two species. Thus the reported capability of inhabiting all substrates of C. verrucosa sensu lato (Ramos-esplá et al., 2005; Tatian et al., 1998) would be mainly a C. verrucosa B trait, and this species could present a longer larval stage and dispersal potential than its counterpart C. verrucosa sp. A . Despite been speculative hypothesis, they drive the attention to the fact that the reported variability on many biological and ecological traits for C. verrucosa sensu lato can be due to added characteristics of two species rather to an actual variability of the species.