Historical biogeography of the Gondwanan freshwater genus Boeckella (Crustacea): Timing and modes of speciation in the Southern Hemisphere

Abstract

Aim We investigated evolutionary relationships and biogeographical patterns within the genus Boeckella to evaluate (1) whether its current widespread distribution in the Southern Hemisphere is due to recent long‐distance dispersal or long‐term diversification; and (2) the age and origin of sub‐Antarctic and Antarctic Boeckella species, with particular focus on the most widely distributed species: Boeckella poppei. Location South America, sub‐Antarctic islands, maritime Antarctica, continental Antarctica and Australasia. Methods To reconstruct phylogenetic patterns of Boeckella, we used molecular sequence data collected from 12 regions and applied Bayesian and Maximum Likelihood analyses using multiple loci. We also estimated divergence times and reconstructed ancestral ranges using two different models of species evolution. Results Phylogenetic analyses and divergence time estimates suggested that Boeckella originated on the Gondwanan supercontinent and initially split into two main clades during the late Cretaceous (ca. 80 Ma). The first clade diversified in Australasia, and the second clade is currently distributed in South America, various sub‐Antarctic islands and Antarctica. Dispersal from South America to the Kerguelen and Crozet archipelagos occurred during the Eocene/Oligocene (B. vallentini) and in the late Pliocene (B. brevicaudata), while South Georgia and the maritime Antarctic were likely colonized during the late Pleistocene (B. poppei). Main conclusions Boeckella has a Gondwanan origin, with further diversifications after the physical separation of the continental landmasses. Extant populations of Boeckella from the Scotia Arc islands and Antarctic Peninsula originated from South America during the Pleistocene, suggesting that original Antarctic Gondwanan lineages did not survive repeated glacial cycles during the Quaternary ice ages. A continuous decline in the species accumulation rate is apparent within the genus as the early Eocene, suggesting that Boeckella diversification may have decreased due to progressive cooling throughout the Cenozoic era.

Survivors and colonizers: Contrasting biogeographic histories reconciled in the Antarctic freshwater copepod Boeckella poppei

“…Based on the interpretation of outgroup specimens, previous phylogenetic and ancestral area reconstruction analyses of the genus (Maturana et al, 2021), divergence time estimates suggest B. poppei originated from an SA ancestor, with Antarctic diversification during the mid-Pleistocene at 994,090 YBP (95% height posterior density (95HPD): 1.5 M-586,090 YBP). This period included an early major branching event with strong PP support (PP = 1) between a clade containing SSI 1 and SOI specimens (hereafter clade A, Figure 3) and a clade containing the SSI 2 , AP, SG, and SA specimens (hereafter clade B, Figure 3).…”

“…Boeckella individuals were collected from lakes, ponds and other freshwater bodies using a zooplankton net (200 μm pore diameter) within five sampling areas (i-v) and three biogeographic regions (1-3): (1) maritime Antarctic (MA), including: (i) South Shetland Islands (62°S 61°W; SSI), (ii) Antarctic Peninsula (67°S 67°W; AP), and (iii) South Orkney Islands (60°S 45°W; SOI); (2) sub-Antarctic: (iv) South Georgia (54°S 36°W; SG); and (3) (v) southern South America (51°S 72°W; SA; Figure 1; for more detail see Supplementary Table S1). We used only sampling sites that contained specimens belonging to the "nominal B. poppei" clade representing the type species, as identified by Maturana et al (2021). Subsequent morphological and molecular analyses were conducted following Maturana et al (2021).…”

“…We used only sampling sites that contained specimens belonging to the "nominal B. poppei" clade representing the type species, as identified by Maturana et al (2021). Subsequent morphological and molecular analyses were conducted following Maturana et al (2021).…”

“…Three DNA loci were amplified: one segment of the fast-evolving mitochondrial cytochrome c oxidase subunit I (cox1), and two slower-evolving segments of the nuclear DNA (nucDNA) 28S rRNA gene and the Internal Transcribed Spacers 1 and 2 (ITS hereafter). For molecular protocols we followed Maturana et al (2021) (see Appendix S2, S3 in Data Source). Sequences were manually quality controlled, examined, assembled, and edited using GENEIOUS 10.2.2 (Kearse et al, 2012).…”

“…The GTR + Ι + Γ model was used for cox1 and the HKY model for 28S rRNA and ITS. For analyses of cox1, 28S, and ITS, the tree was rooted with B. vallentini, based on previous phylogenetic reconstructions in Boeckella (Maturana et al, 2021).…”