The age and diversification of metacrangonyctid subterranean amphipod crustaceans revisited

Mitogenomics of Cladocera (Branchiopoda): Marked gene order rearrangements and independent predation roots

Cladocera (Crustacea: Branchiopoda) is a key group of invertebrates. Despite a long history of phylogenetic research, relationships within this group remain disputed. We here provide new insights based on 15 new mitochondrial genomes obtained from high-throughput sequencing (HTS) and 40 mitogenomes extracted from published HTS datasets. Together with 25 mitogenomes from GenBank, we generated a matrix of 80 mitogenomes, 44 of them belonging to Cladocera. We also obtained a matrix with 168 nuclear orthologous genes to further assess the phylogenetic result from mitogenomes based on published data and one new HTS data ofLeptodora. Maximum likelihood and Bayesian phylogenetic analyses recovered all Branchiopoda orders as monophyletic and supported a sister-group relationship between Anomopoda and Onychopoda, making the taxon Gymnomera paraphyletic and supporting an independent origin of predatory Haplopoda and Onychopoda. The nuclear phylogeny and topological tests also support Gymnomera as paraphyletic, and the nuclear phylogeny strongly supports a sister-group relationship between Ctenopoda and Haplopoda. We provide a fossil-calibrated time tree, congruent with a Carboniferous origin for Cladocera and a subsequent diversification of the crown group of Anomopoda, Onychopoda, and Ctenopoda, at least in the Triassic. Despite their long evolutionary history, non-Cladoceran Branchiopoda exhibited high mitogenome structural stability. On the other hand, 21 out of 24 gene rearrangements occurred within the relatively younger Cladocera. We found the differential base compositional skewness patterns between Daphnia s.s. and Ctenodaphnia, which might be related to the divergence between these taxa. We also provide evidence to support the recent finding that Spinicaudata possesses mitogenomes with inversed compositional skewness without gene rearrangement. Such a pattern has only been reported in Spinicaudata.

Mitogenome phylogenetics in the genus Palaemon (Crustacea: Decapoda) sheds light on species crypticism in the rockpool shrimp P. elegans

The genus Palaemon comprises worldwide marine and freshwater shrimps and prawns, and some of them are ecologically or commercially important species. Palaemon is not currently a monophyletic group, so phylogenetics and systematics are constantly changing. Species crypticism has been pointed out in several Palaemon species, being the clearest evidence in the European rockpool shrimp P. elegans. Here we sequenced and described seven European Palaemon mitochondrial genomes. The mitochondrial protein-coding genes were used, along with those of three other Palaemon species, to perform mitogenome phylogenetic analyses to clarify the evolutionary relationships within the genus, and particularly to shed light on the cryptic species found within P. elegans. The Messinian Salinity Crisis (5.3-5.9 Ma, late Miocene) was proposed to be the origin of this cryptic species and it was used as aged constraint for calibration analysis. We provide the largest and the first time-calibrated mitogenome phylogeny of the genus Palaemon and mitogenome substitution rate was estimated (1.59% per million years) in Decapoda for the first time. Our results highlighted the need for future systematics changes in Palaemon and crypticism in P. elegans was confirmed. Mitochondrial genome and cox1 (1.41%) substitution rate estimates matched those published elsewhere, arguing that the Messinian Salinity Crisis was a plausible event driving the split between P. elegans and its cryptic species. Molecular dating suggested that Pleistocene glaciations were likely involved in the differentiation between the Atlantic and Mediterranean populations of P. elegans. On the contrary, the divergence between the Atlantic and Mediterranean populations of the common littoral shrimp P. serratus was greater and dated to be much older (4.5-12.3 Ma, Plio-Miocene), so we considered that they could represent two separated species. Therefore, species crypticism in the genus Palaemon seems to be a common phenomenon.

Near-complete phylogeny of extant Crocodylia (Reptilia) using mitogenome-based data

Species of the order Crocodylia are mostly large, predatory and semi-aquatic reptiles. Crocodylia, the closest living relatives of birds, first appeared in the Late Cretaceous period. In the present study, the complete mitochondrial (mt) genomes of 19 Crocodylia species, including two species (Melanosuchus niger and Caiman yacare) that have not been previously sequenced for mitogenomes, were processed through Illumina sequencing to offer genetic resources and compare with the mitogenomes of Crocodylia species reported previously. In addition, a high-resolution phylogenetic tree of nearly all current recognized species of Crocodylia is constructed based on mitogenomic data. Phylogenetic analyses support monophyly of three families: Alligatoridae (four genera: Alligator, Caiman, Melanosuchus and Paleosuchus), Crocodylidae (three genera: Crocodylus, Mecistops and Osteolaemus) and Gavialidae (two genera: Gavialis and Tomistoma). The tree topology is generally similar to previous studies. Molecular dating suggests that the first split within Crocodylia date back to the Upper Cretaceous (approx. 86.75 Mya). The estimated time to the most recent common ancestor (TMRCA) of Alligatoridae is 53.33 Mya and that of Crocodylidae and Gavialidae is 50.13 Mya, which might be closely linked to climate changes during the Late Palaeocene and Early Eocene. Additionally, this study proves that the diversification rate within Crocodylia began to increase from the Late Eocene (about 36 Mya) and two diversification peak periods of Crocodylia (0–10 Mya and 10–20 Mya) are disclosed, which is roughly consistent with the estimated crocodylian species richness through time. Combining all these clues, we can suggest that climate fluctuation may have played a decisive role in the speciation of Crocodylia.