Sources of Signal in 62 Protein-Coding Nuclear Genes for Higher-Level Phylogenetics of Arthropods

BACKGROUND: This study aims to investigate the strength of various sources of phylogenetic information that led to recent seemingly robust conclusions about higher-level arthropod phylogeny and to assess the role of excluding or downweighting synonymous change for arriving at those conclusions. METHODOLOGY/PRINCIPAL FINDINGS: The current study analyzes DNA sequences from 68 gene segments of 62 distinct protein-coding nuclear genes for 80 species. Gene segments analyzed individually support numerous nodes recovered in combined-gene analyses, but few of the higher-level nodes of greatest current interest. However, neither is there support for conflicting alternatives to these higher-level nodes. Gene segments with higher rates of nonsynonymous change tend to be more informative overall, but those with lower rates tend to provide stronger support for deeper nodes. Higher-level nodes with bootstrap values in the 80% - 99% range for the complete data matrix are markedly more sensitive to substantial drops in their bootstrap percentages after character subsampling than those with 100% bootstrap, suggesting that these nodes are likely not to have been strongly supported with many fewer data than in the full matrix. Data set partitioning of total data by (mostly) synonymous and (mostly) nonsynonymous change improves overall node support, but the result remains much inferior to analysis of (unpartitioned) nonsynonymous change alone. Clusters of genes with similar nonsynonymous rate properties (e.g., faster vs. slower) show some distinct patterns of node support but few conflicts. Synonymous change is shown to contribute little, if any, phylogenetic signal to the support of higher-level nodes, but it does contribute nonphylogenetic signal, probably through its underlying heterogeneous nucleotide composition. Analysis of seemingly conservative indels does not prove useful. CONCLUSIONS: Generating a robust molecular higher-level phylogeny of Arthropoda is currently possible with large amounts of data and an exclusive reliance on nonsynonymous change

Evolution of pycnogonid life history traits

The Pycnogonida is a class of arthropods with interesting life histories. Pycnogonids prey on hydroids and some invade hydranths while larvae. Males brood the eggs and larvae hatch as protonymphons. Questions relating to the evolution of life history characteristics were addressed. Evolutionary relationships were poorly understood. It was necessary to determine the relationships within the Pycnogonida and compared to other arthropods. Twenty-four morphological characters were coded for twenty-three pycnogonid genera and one fossil ancestor, Palaeoisopus problematicus. A branch and bound analysis resulted in fifteen most parsimonious trees. The Nymphonidae were found to be basal. The Ammotheidae were paraphyletic and led to two clades. The first contained the Callipallenidae, and Phoxichilidiidae. The second contained the remaining pycnogonids. A phylogeny was also compiled using sequences of the D3 expansion segments of 28S rDNA. This resolved relationships of sampled families as follows (Ammotheidae + ((Nymphonidae + Colossendeidae) + (Endeididae + (Pycnogonidae + Phoxichilidiidae)))). The Ammotheidae was found to be paraphyletic and basal. The results from the D3 region yielded perplexing relationships when compared with morphology. Phoxichilidium tubulariae Lebour 1947 is a valid species. It appeared to be specialists on the hydroid Tubularia larynx. Annual population dynamics of P. tubulariae were seasonal. Density of adult animals was highest in mid to late summer with reproduction being greatest in July and August. The abundance of pycnogonids peaked as the hydroid population declined. Some populations were shown to have two generations. Adult migration may play a larger role in the distribution of this species than larval dispersal. Phoxichilidium tubulariae had an atypical protonymphon type developmental mode that reduced the typical number of molts, and developed rapidly in the gastrovascular cavities of the host. It decreased developmental time from 35--40 days to 15--20 days. This was adapted to exploit the seasonal abundance of Tubularia larynx. The male looped the egg mass over his oviger. The larvae hatched, infected the hydroid, and developed inside the gastrovascular cavity of T. larynx. The larvae developed for several molts and then hatched, destroying the hydranth. The ancestral pycnogonid stock were external parasites. The internalization of the larval stages appeared to have happened at least twice

The pycnogonid (Endeis mollis Carpenter, 1904) associated with hydroids from the inshore waters of Visakhapatnam, India

The sea spider or pycnogonid, identified as Endeis mollis Carpenter, 1904 were isolated from hydroids colonized for over 45 days on a large floating cage installed at Visakhapatnam inshore area at a depth of 10-12 m. More than 50% of the population consisted of males carrying egg mass. Occurrence of E. mollis off Visakhapatnam as well as their association with hydroids in Indian waters is reported for the first time

First record of Pentapycnon geayi Bouvier, 1911 (Pycnogonida: Pycnogonidae) in the state of Ceará, northeastern Brazil

Contributions to the knowledge of the Pycnogonida fauna along the Brazilian coast are scarce. There are only 60 records for the coast of Brazil, and reports of this fauna are notably discontinuous. This is the first record of Pentapycnon geayi Bouvier, 1911 (Pycnogonida: Pycnogonidae) in the state of Ceará. This study adds a new bathymetric record and fills a gap in the distribution of P. geayi along the Brazilian coast. This report also provides important new data for Ceará because the diversity of Pycnogonida in this state is practically unknown

Pycnogonida (Arthropoda) from Museu de Ciências Naturais, Rio Grande do Sul, Brazil

Five species were identified in the studied collection: Colossendeis megalonyx Hoek, 1881, first record for Uruguay, Ascorhynchus corderoi du Bois-Reymond Marcus, 1952 and Pallenopsis candidoi Mello-Leitão, 1949, with extended ranges, Pallenopsis patagonica (Hoek, 1881), a species complex recently analysed with molecular data and Ammothea tetrapoda, recorded previously for Uruguayan waters. Our study clarifies records based on morphology, provides new data on distributions and species ranges and correlates species with ecological conditions

BODY SIZE IN ANTARCTIC AND TEMPERATE SEA SPIDERS: THE ROLE OF TEMPERATURE AND OXYGEN

Ph.D

Austropallene halanychi sp. nov., a new species of sea spider (Pycnogonida, Callipallenidae) from the Ross Sea, Antarctica

Here we present Austropallene halanychi sp. nov., a new species of pycnogonid within the family Callipallenidae (Pycnogonida), collected from the Ross Sea, Antarctica. While retaining key morphological features known for the genus Austropallene Hodgson, 1915a, the new species is distinguished from congeners by its much larger size, along with the combined absence of a denticle on the inner surface of the fixed finger of the chelifore claw along with the presence of small conical outgrowths where the fixed finger of the chelifore claw meets the movable finger on both the dorsal and ventral sides, and also the ability to fully close the chelifore claw. Additionally, the complete mitochondrial genome of A. halanychi is consistent with other members of the genus Austropallene in terms of gene order and directionality. A phylogenetic tree consisting of mitochondrial protein-coding gene data places A. halanychi as sister to Austropallene cornigera (Möbius, 1902). Additionally, a phylogenetic tree constructed using partial COI data from other callipallenids placed the new species in a clade containing the genus Austropallene. The combination of molecular data in addition to key morphological differences from similar species in the genus leaves no doubt that the new taxon is a new Antarctic species of Austropallene

Combining morphological and genomic evidence to resolve species diversity and study speciation processes of the Pallenopsis patagonica (Pycnogonida) species complex

Background: Pallenopsis patagonica (Hoek, 1881) is a morphologically and genetically variable sea spider species whose taxonomic classification is challenging. Currently, it is considered as a species complex including several genetic lineages, many of which have not been formally described as species. Members of this species complex occur on the Patagonian and Antarctic continental shelves as well as around sub-Antarctic islands. These habitats have been strongly influenced by historical large-scale glaciations and previous studies suggested that communities were limited to very few refugia during glacial maxima. Therefore, allopatric speciation in these independent refugia is regarded as a common mechanism leading to high biodiversity of marine benthic taxa in the high-latitude Southern Hemisphere. However, other mechanisms such as ecological speciation have rarely been considered or tested. Therefore, we conducted an integrative morphological and genetic study on the P. patagonica species complex to i) resolve species diversity using a target hybrid enrichment approach to obtain multiple genomic markers, ii) find morphological characters and analyze morphometric measurements to distinguish species, and iii) investigate the speciation processes that led to multiple lineages within the species complex. Results: Phylogenomic results support most of the previously reported lineages within the P. patagonica species complex and morphological data show that several lineages are distinct species with diagnostic characters. Two lineages are proposed as new species, P. aulaeturcarum sp. nov. Dömel & Melzer, 2019 and P. obstaculumsuperavit sp. nov. Dömel, 2019, respectively. However, not all lineages could be distinguished morphologically and thus likely represent cryptic species that can only be identified with genetic tools. Further, morphometric data of 135 measurements showed a high amount of variability within and between species without clear support of adaptive divergence in sympatry. Conclusions: We generated an unprecedented molecular data set for members of the P. patagonica sea spider species complex with a target hybrid enrichment approach, which we combined with extensive morphological and morphometric analyses to investigate the taxonomy, phylogeny and biogeography of this group. The extensive data set enabled us to delineate species boundaries, on the basis of which we formally described two new species. No consistent evidence for positive selection was found, rendering speciation in allopatric glacial refugia as the most likely model of speciation

Genetic data support independent glacial refugia and open ocean barriers to dispersal for the Southern Ocean sea spider Austropallene cornigera (Möbius, 1902)

The diversity and distribution of Antarctic life has been strongly influenced by climatic events, in particular by large scale extension of ice sheets onto the continental shelf during repeated glacial cycles. It has been suggested that populations of benthic marine biota in the Antarctic were limited to very few refugia because the Antarctic shelf was covered with ice. Using the broadly-distributed pycnogonid Austropallene cornigeraas a model, in this study we tested different hypotheses for possible locations of glacial refugia ( ex situon the peri-Antarctic islands or in situon the Antarctic shelf). We sampled 64 individuals of A. cornigerafrom peri-Antarctic islands, the Weddell Sea and East Antarctica. The phylogeographic structure was analysed using partial sequences of the nuclear ribolomal genes 18S and 28S and the mitochondrial cytochrome coxidase subunit I gene (COI). The 18S and 28S sequences were highly conserved. Sequences of the COI were variable and revealed highest haplotype diversity for populations on the Antarctic shelf and lowest for the population from the remote island of Bouvetøya. In addition, the data showed clear genetic distances between the island and shelf populations. Our data are consistent with the hypothesis of survival in situ. The results also suggest that gene flow within A. cornigerais limited, hinting at possible speciation processes acting independently on the Antarctic continental shelf and the peri-Antarctic islands