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.

A species-specific nematocide that results in terminal embryogenesis.

Nematode-insect interactions are ubiquitous, complex and constantly changing as the host and nematode coevolve. The entomophilic nematode Pristionchus pacificus is found on a myriad beetle species worldwide, although the molecular dynamics of this relationship are largely unknown. To better understand how host cues affect P. pacificus embryogenesis, we characterized the threshold of sensitivity to the pheromone (Z)-7-tetradecen-2-one (ZTDO) by determining the minimum exposure duration and developmental window that results in P. pacificus embryonic lethality. We found early-stage embryos exposed to volatile ZTDO for as few as 4 h all display terminal embryogenesis, characterized by punctuated development up to 48 h later, with abnormal morphology and limited cavity formation. To determine if the pheromone arrests pre-hatching development by suffocating or permeabilizing the eggshells, we raised embryos under anoxic conditions and also examined eggshell permeability using the lipophilic dye FM4-64. We found that asphyxiating the embryos arrested embryogenesis in a reversible manner but did not phenocopy the effects of ZTDO exposure, whereas the ZTDO-induced disruption of embryogenesis did correlate with increased eggshell permeability. The effects of ZTDO are also highly specific, as other lipid insect compounds do not produce any detectable embryocidal effect. The high specificity and unusual teratogenic effect of ZTDO may be important in mediating the host-nematode relationship by regulating P. pacificus development.

Trapping and imaging of micron-sized embryos using dielectrophoresis.

Development of dielectrophoretic (DEP) arrays for real-time imaging of embryonic organisms is described. Microelectrode arrays were used for trapping both embryonated eggs and larval stages of Antarctic nematode Panagrolaimus davidi. Ellipsoid single-shell model was also applied to study the interactions between DEP fields and developing multicellular organisms. This work provides proof-of-concept application of chip-based technologies for the analysis of individual embryos trapped under DEP force.

Unusual intestinal lamellae in the nematode Rhabditophanes sp. KR3021 (Nematoda: Alloinematidae).

The free-living nematode Rhabditophanes sp. has recently been placed in a clade of animal parasites and may be a unique example of a reversal to a nonparasitic lifestyle. Detailed morphological analysis of the intestine reveals the unusual and unique structure of splitting microlamellae forming a meshwork with cavities along the entire intestinal tract. Secretion vesicles were observed along the whole tract and along the length of the lamellae. It is suggested that these lamellae are adaptations to a different digestive strategy where low food availability and a low absorption surface are compensated for by maximizing the nutrient uptake efficiency along the entire length of the intestine. The likely reversal to a free-living life cycle may have caused drastic changes in diet, providing the necessary driving forces to such morphological changes.