3.3. Phylogenetic analysis and divergence time
The phylogenetic trees based on both molecular markers (nuclear and
mitochondrial), using the Maximum Likelihood and Bayesian Inference
approach show two well-supported, reciprocally monophyletic and
congruent groups of samples (posterior probability, PP ≥ 0.99; bootstrap
probability, BP = 99) (Fig. 2). Group “A” was more widely distributed
along the Antarctic Peninsula, whereas group “B” was distributed in
Weddell Sea, Potter Cove, Palmer Station, Paradise Bay, and Rothera
Station. Moreover, the phylogeny built with the nuclear gene separated
four extra groups; one cluster comprises exclusively individuals from
Burdwood Bank/MPA Namuncurá (PP 1, BP 100), a second group with two
samples from Potter Cove (PP 1, BP 98), a third branch with only one
sample from Burdwood Bank/MPA Namuncurá, and a fourth group constituted
by samples from Scotia Sea (PP 1, BP 99).
The two congruent clusters defined by the nuclear and mitochondrial
phylogenetic trees were grouping the sequences in the same way as ABGD
analysis and the haplotype network. However, two individuals were
assigned to different groups depending on which marker was used,
mitochondrial or nuclear. On one hand, the individual 291 from Potter
Cove, was assigned to group A in 18S species delimitation, but to group
B for COI. On the other hand, the individual 116 from Palmer Station was
assigned to group B in 18S species delimitation, but to group A for COI.
This crossed pattern is incompatible with congruent diversification of
mitochondrial and nuclear genes, some hypothetical explanations for this
result are discussed below (see discussion 4.4 section).
The estimation of divergence time among groups A and B, using COI
sequences, was calculated between 3.58 Ma (95% High Posterior Density
[HPD]: 2.331 - 4.935 Ma) and 2.20 Ma (95% High Posterior Density
[HPD]: 1.423 - 3.028Ma).