5. Conclusions
This work provides new
insights to understand the evolution of molecules and morphology at the
same time. The evidence presented here allowed to delimitate two species
within C. verrucosa : two monophyletic groups were obtained;
congruent mitochondrial, nuclear species delimitation was demonstrated.
The magnitude of difference between the two distinguished species was
similar to those among undisputed species pairs, and both species were
distributed in sympatry. Morphological analysis suggests that the
presence of a basal disc could be a morphological feature distinguishing
the species. Nominal C. verrucosa sensu lato are
distributed all around Antarctica and in the south of South America.
However, the true extent of the species distribution area is obscured by
the existence of two, possibly more species that have up until now been
mistaken as a single species. Incongruent mitochondrial-nuclear
phylogenies might be explained by different hypotheses as
introgression/hybridization or ILS. Boundaries between sympatric species
are maintained by barriers to gene flow, these restrictions may not be
uniform in space, time or across the genome. In fact, it has been
proposed that these barriers are semipermeable and speciation under gene
flow is possible (Nosil, 2008).
Further analysis
employing several nuclear loci and an extended geographical sampling,
would help to elucidate the evolutionary story of this broadly
distributed Antarctic ascidian species. Patterns regarding genetic and
morphological differentiation that are being underestimated or not
registered systematically, can lead to important misunderstanding of
species distribution patterns related to adaptation, habitat preference,
competition and response to climate change. Our results once again
emphasize that species identities, even for highly abundant and
well-studied species on small local scales, must be assessed rather than
assumed.
6. List of
abbreviations
ABGD: Automatic barcode
gap
APF: Antarctic Polar Front
ILS: Incomplete lineage
sorting
ACC: Antarctic
circumpolar current
MYA: Million years ago
7.
Acknowledgements
The authors want to particularly thank the members of the Carlini
(former Jubany) - Dallmann staff and Andrea Eschbach for kind assistance
in laboratory work. Logistic and financial support was provided by
Instituto Antártico Argentino (IAA), Alfred Wegener Institute (AWI),
CONICET, IMCONet [FP7 IRSES (International Research Staff Exchange
Scheme), action no. 319718], Short Term Grant 2019 Scholarship of
Deutscher Akademischer Austauschdienst [German Academic Exchange
Service (DAAD) Reference number: 91701637], and Lerner-Gray Memorial
Fund of the American Museum of Natural History. The authors are
grateful to the crew of the R/V “Puerto Deseado” and Gisela Morán for
help in sampling. This work was partially supported by the Área Marina
Protegida Namuncurá (Ley 26.875). Particular thank to Marcos Tatián who
provided scientific support and guidance in the morphological analysis,
and Cristian Lagger who provided the underwater photos.