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.