Different roads to form the same gut in nematodes

Redescription and molecular characterisation of Panagrolaimus labiatus (Kreis, 1929) Andrássy, 1960 (Rhabditida, Panagrolaimidae) from India and proposal of P. burdwanensis Chaturvedi & Khera, 1979 as a junior synonym of P. labiatus

Two populations of bacterial-feeding nematodes belonging to the genus Panagrolaimus were recovered from infected slugs collected from gardens, agricultural areas and nurseries of the Baghpat district (India). Initial morphological characterisation of these populations show that they resemble Panagrolaimus labiatus from China and Bulgaria, P. burdwanensis from India, and P. cf. labiatus from Iran. To clarify taxonomical affiliations, we morphologically compared these two new populations to those recovered from China, Bulgaria, India and Iran. In addition, we sequenced several taxonomically relevant genes and used them to reconstruct phylogenetic relationships. Our results show that the two Indian populations do not significantly differ morphologically from the specimens used to describe the species P. labiatus, and also do not differ among them. We only observed small variations in the size of males and females. In addition, morphologically, the nematodes used to describe P. burdwanensis are very similar to the nematodes used to describe P. labiatus and to the two populations described in this study. Therefore, we propose P. burdwanensis as a junior synonym of P. labiatus and that the two populations recovered from India belong to the P. labiatus species. Phylogenetic analyses using the nucleotide sequences of the internal transcribed spacer (ITS), 18S and 28S rRNA gene sequences support our morphological conclusions. Our study therefore provides a detailed morphological description and molecular marker 18S and 28S gene sequences that can support future taxonomical studies of this species and highlights the importance of using both molecular and morphological data for the proper description of soil microfauna.

The gene regulatory program of Acrobeloides nanus reveals conservation of phylum-specific expression

The evolution of development has been studied through the lens of gene regulation by examining either closely related species or extremely distant animals of different phyla. In nematodes, detailed cell- and stage-specific expression analyses are focused on the model Caenorhabditis elegans, in part leading to the view that the developmental expression of gene cascades in this species is archetypic for the phylum. Here, we compared two species of an intermediate evolutionary distance: the nematodes C. elegans (clade V) and Acrobeloides nanus (clade IV). To examine A. nanus molecularly, we sequenced its genome and identified the expression profiles of all genes throughout embryogenesis. In comparison with C. elegans, A. nanus exhibits a much slower embryonic development and has a capacity for regulative compensation of missing early cells. We detected conserved stages between these species at the transcriptome level, as well as a prominent middevelopmental transition, at which point the two species converge in terms of their gene expression. Interestingly, we found that genes originating at the dawn of the Ecdysozoa supergroup show the least expression divergence between these two species. This led us to detect a correlation between the time of expression of a gene and its phylogenetic age: evolutionarily ancient and young genes are enriched for expression in early and late embryogenesis, respectively, whereas Ecdysozoa-specific genes are enriched for expression during the middevelopmental transition. Our results characterize the developmental constraints operating on each individual embryo in terms of developmental stages and genetic evolutionary history.

Embryogenesis of Romanomermis culicivorax: an alternative way to construct a nematode

The current picture of embryonic development in nematodes is essentially shaped by Caenorhabditis elegans and its close relatives. As their pattern of embryogenesis is rather similar, it is often considered to be representative for the taxon Nematoda as a whole. Here we give for the first time a comprehensive description of embryonic development in an ancestrally diverged nematode. Romanomermis culicivorax differs strikingly from C. elegans with respect to cell division pattern, spatial arrangement of blastomeres and tissue formation. Our study reveals a number of unexpected phenomena. These include (i) unique polar interphase microtubule caps forming in early blastomeres destined to undergo asymmetric cleavages, suggesting the presence of a so far undescribed MTOC; (ii) embryonic cell lineages of reduced complexity with predominantly monoclonal sublineages, generating just a single tissue type; (iii) construction of major parts of the body from duplicating building blocks consisting of rings of cells, a pattern showing some resemblance to segmentation; (iv) prominent differences in cell fate assignment which can be best explained with a global shift affecting all somatic founder cells. In summary, our data indicate that during nematode evolution massive alterations in the developmental program took place of how to generate a juvenile.

The small subunit (SSU) ribosomal (r) RNA gene as a genetic marker for identifying infective 3rd juvenile stage Angiostrongylus cantonensis

We have developed a molecular method using PCR-direct sequencing to identify the infective 3rd juvenile stage of Angiostrongylus cantonensis, a nematode parasite of rodents that can accidentally infect humans and cause eosinophilic meningitis. We demonstrate that the 5' end of the small subunit (SSU) ribosomal (r) RNA gene is a suitable marker to identify A. cantonensis and distinguish it from other closely related Angiostrongylus species. When the SSU rRNA marker was employed on nematode populations extracted from the black slug Laevicaulis altae collected in 2 test sites in the Philippines, the infective 3rd juvenile stage A. cantonensis was detected without difficulty. The phylogenetic position of other non-angiostrongylid nematodes isolated was also determined. The molecular technique developed in this study provides a rapid and accurate method for the identification of A. cantonensis when morphological identification proves difficult or inadequate.

Cellular pattern formation, establishment of polarity and segregation of colored cytoplasm in embryos of the nematode Romanomermis culicivorax

We have begun to analyze the early embryogenesis of Romanomermis culicivorax, an insect-parasitic nematode phylogenetically distant to Caenorhabditis elegans. Development of R. culicivorax differs from C. elegans in many aspects including establishment of polarity, formation of embryonic axes and the pattern of asymmetric cleavages. Here, a polarity reversal in the germline takes place already in P(1) rather than P(2), the dorsal-ventral axis appears to be inverted and gut fate is derived from the AB rather than from the EMS blastomere. So far unique for nematodes is the presence of colored cytoplasm and its segregation into one specific founder cell. Normal development observed after experimentally induced abnormal partitioning of pigment indicates that it is not involved in cell specification. Another typical feature is prominent midbodies (MB). We investigated the role of the MB region in the establishment of asymmetry. After its irradiation the potential for unequal cleavage in somatic and germline cells as well as differential distribution of pigment are lost. This indicates a crucial involvement of this region for spindle orientation, positioning, and cytoplasmic segregation. A scenario is sketched suggesting why and how during evolution the observed differences between R. culicivorax and C. elegans may have evolved.