Phylogenomic analysis of echinoderm class relationships supports Asterozoa

While some aspects of the phylogeny of the five living echinoderm classes are clear, the position of the ophiuroids (brittlestars) relative to asteroids (starfish), echinoids (sea urchins) and holothurians (sea cucumbers) is controversial. Ophiuroids have a pluteus-type larva in common with echinoids giving some support to an ophiuroid/echinoid/holothurian clade named Cryptosyringida. Most molecular phylogenetic studies, however, support an ophiuroid/asteroid clade (Asterozoa) implying either convergent evolution of the pluteus or reversals to an auricularia-type larva in asteroids and holothurians. A recent study of 10 genes from four of the five echinoderm classes used ‘phylogenetic signal dissection’ to separate alignment positions into subsets of (i) suboptimal, heterogeneously evolving sites (invariant plus rapidly changing) and (ii) the remaining optimal, homogeneously evolving sites. Along with most previous molecular phylogenetic studies, their set of heterogeneous sites, expected to be more prone to systematic error, support Asterozoa. The homogeneous sites, in contrast, support an ophiuroid/echinoid grouping, consistent with the cryptosyringid clade, leading them to posit homology of the ophiopluteus and echinopluteus. Our new dataset comprises 219 genes from all echinoderm classes; analyses using probabilistic Bayesian phylogenetic methods strongly support Asterozoa. The most reliable, slowly evolving quartile of genes also gives highest support for Asterozoa; this support diminishes in second and third quartiles and the fastest changing quartile places the ophiuroids close to the root. Using phylogenetic signal dissection, we find heterogenous sites support an unlikely grouping of Ophiuroidea + Holothuria while homogeneous sites again strongly support Asterozoa. Our large and taxonomically complete dataset finds no support for the cryptosyringid hypothesis; in showing strong support for the Asterozoa, our preferred topology leaves the question of homology of pluteus larvae open.

A novel ontogenetic pathway in hybrid embryos between species with different modes of development

To investigate the bases for evolutionary changes in developmental mode, we fertilized eggs of a direct-developing sea urchin, Heliocidaris erythrogramma, with sperm from a closely related species, H. tuberculata, that undergoes indirect development via a feeding larva. The resulting hybrids completed development to form juvenile adult sea urchins. Hybrids exhibited restoration of feeding larval structures and paternal gene expression that have been lost in the evolution of the direct-developing maternal species. However, the developmental outcome of the hybrids was not a simple reversion to the paternal pluteus larval form. An unexpected result was that the ontogeny of the hybrids was distinct from either parental species. Early hybrid larvae exhibited a novel morphology similar to that of the dipleurula-type larva typical of other classes of echinoderms and considered to represent the ancestral echinoderm larval form. In the hybrid developmental program, therefore, both recent and ancient ancestral features were restored. That is, the hybrids exhibited features of the pluteus larval form that is present in both the paternal species and in the immediate common ancestor of the two species, but they also exhibited general developmental features of very distantly related echinoderms. Thus in the hybrids, the interaction of two genomes that normally encode two disparate developmental modes produces a novel but harmonious ontongeny.

The karyotype and morphology of Chironomus brevidentatus sp. n. (Dipt., Chironomidae). A species with a "salinarius type" larva from northern Finland

The karyotype and morphology of Chironomus brevidentatus sp. n. has been described from the Lokka reservoir in northern Finland. The species belongs to the thummi complex with the chromosome arm combination AB, CD, EF and G. The relationships between the karyotypes of C.brevidentatus, C. major Wülker & Butler, C. neocorax Wülker and Butler, C. tardus Butler, and "Chironomus sp. 2n = 6, Lokka" are described. The external morphology has been compared with the suggested nearest relatives or otherwise similar species. The known ecological conditions in the type locality are discussed.

A Quantitative Study of Economically Important Parasites of Walleye Pollock (Theragra chalcogramma) from British Columbian Waters and Effects of Postmortem Handling on Their Abundance in the Musculature

The identity of parasites infecting the musculature and body cavity (including external surfaces of viscera) of walleye pollock (Theragra chalcogramma) collected from two British Columbian localities, their distribution within the host's musculature, and the influence of host sex, length, and method of postmortem handling on parasite prevalence and abundance were determined. Three parasite species were found only in the musculature (Pleistophora sp., pseudophyllidean plerocercoids, and Phocanema decipiens (Krabbe, 1878) larva), two species only in the body cavity (Contracaecum-type larva and Hysterothylacium aduncum (Rudolphi, 1802) larva), and two species in both sites (Anisakis simplex (Rudolphi, 1809) larva and Nybelinia surmenicola Okada in Dollfus, 1929 plerocercoid). In the musculature most A. simplex and N. surmenicola infected the region surrounding the body cavity, with A. simplex being distributed more anteriorly, while most pseudophyllidean plerocercoids occurred in the caudal region. No preferred site of infection was observed for P. decipiens larvae. No statistically significant differences in infection with any of these helminths occurred between right and left body musculature. For pollock of similar lengths, males were more heavily infected than females with larvae of A. simplex and plerocercoids of N. surmenicola. Three species (A. simplex, N. surmenicola, and Pleistophora sp.) occurred with greater prevalence or abundance in larger fish. Storage of eviscerated and round pollock from the Strait of Georgia on ice at 3 °C for periods of up to 7 d did not result in any significant change in the prevalence of infection or abundance of parasites in the musculature. The abundance of these parasites in pollock from Queen Charlotte Sound, northern British Columbia, appears similar to that previously reported for some other areas of the North Pacific Ocean (although not as high as the most heavily infected stocks of the northwest Pacific) while their abundance in fish from the Strait of Georgia, southern British Columbia, appears to be the lowest for any pollock stock examined.Key words: walleye pollock, Theragra chalcogramma; economically important parasites, Pleistophora sp., pseudophyllidean plerocercoids, Nybelinia surmenicola plerocercoid, Phocanema decipiens larva, Contracaecum-type larva, Hysterothylacium aduncum larva, Anisakis simplex larva, British Columbia