Natural variation of outcrossing in the hermaphroditic nematode Pristionchus pacificus.

BACKGROUND: Evolution of selfing can be associated with an increase in fixation of deleterious mutations, which in certain conditions can lead to species extinction. In nematodes, a few species evolved self-fertilization independently, making them excellent model systems to study the evolutionary consequences of this type of mating system. RESULTS: Here we determine various parameters that influence outcrossing in the hermaphroditic nematode Pristionchus pacificus and compare them to the better known Caenorhabditis elegans. These nematode species are distinct in terms of genetic diversity, which could be explained by differences in outcrossing rates. We find that, similarly to C. elegans, P. pacificus males are generated at low frequencies from self-fertilizing hermaphrodites and are relatively poor mating partners. Furthermore, crosses between different isolates reveal that hybrids have lower brood sizes than the pure strains, which is a sign of outbreeding depression. In contrast to C. elegans, P. pacificus has lower brood sizes and the male X-bearing sperm is able to outcompete the X-nullo sperm. CONCLUSION: The results indicate that there is no evidence of any selection acting very strongly on P. pacificus males.

The nematode Pristionchus pacificus (Nematoda: Diplogastridae) is associated with the oriental beetle Exomala orientalis (Coleoptera: Scarabaeidae) in Japan.

Pristionchus pacificus has been developed as a nematode satellite organism in evolutionary developmental biology. Detailed studies of vulva development revealed multiple differences in genetic and molecular control in P. pacificus compared to the model organism Caenorhabditis elegans. To place evolutionary developmental biology in a comprehensive evolutionary context, such studies have to be complemented with ecology. In recent field studies in western Europe and eastern North America we found 11 Pristionchus species that are closely associated with scarab beetles and the Colorado potato beetle. However, P. pacificus was not commonly found in association with scarab beetles in these studies. Here, we describe the results of a similar survey of scarab beetles in Japan. Pristionchus pacificus was the most common Pristionchus species on scarab beetles in Japan, with 40 out of 43 (93%) isolates. The other Pristionchus isolates represent three novel species, which we refer to as Pristionchus sp. 11, Pristionchus sp. 14, and Pristionchus sp. 15. Thirty-seven of the established P. pacificus strains were found on the oriental beetle Exomala orientalis. Laboratory studies with the sex pheromone (Z)-7-tetradecen-2-one of the oriental beetle revealed that P. pacificus shows strong olfactory attraction to the beetle's sex pheromone, which provides a potential mechanism for the recognition and interaction of P. pacificus and E. orientalis. Together, this study identifies P. pacificus as the most common Pristionchus nematode in field studies in Japan, identifies E. orientalis as an important host species, and provides the basis for the ecology of P. pacificus.

Long-term time-lapse microscopy of C. elegans post-embryonic development.

We present a microscopy technique that enables long-term time-lapse microscopy at single-cell resolution in moving and feeding Caenorhabditis elegans larvae. Time-lapse microscopy of C. elegans post-embryonic development is challenging, as larvae are highly motile. Moreover, immobilization generally leads to rapid developmental arrest. Instead, we confine larval movement to microchambers that contain bacteria as food, and use fast image acquisition and image analysis to follow the dynamics of cells inside individual larvae, as they move within each microchamber. This allows us to perform fluorescence microscopy of 10-20 animals in parallel with 20 min time resolution. We demonstrate the power of our approach by analysing the dynamics of cell division, cell migration and gene expression over the full ∼48 h of development from larva to adult. Our approach now makes it possible to study the behaviour of individual cells inside the body of a feeding and growing animal.

A locus in Pristionchus pacificus that is responsible for the ability to give rise to fertile offspring at higher temperatures.

Temperature is a stress factor that varies temporally and spatially, and can affect the fitness of cold-blooded organisms, leading to a loss of reproductive output; however, little is understood about the genetics behind the long-term response of organisms to temperature. Here, we approach this problem in the model nematode Pristionchus pacificus by utilising a large collection of natural isolates with diverse phenotypes. From this collection we identify two strains, one from California that can give rise to fertile offspring up to 28°C and one from Japan that is fertile up to 30°C. We show that the optimum temperature and the upper temperature limit for fertility is shifted higher in the Japanese strain suggesting that there is a mechanism that controls the temperature response of fertility across a range of temperatures. By crossing the two strains, and using genetic mapping, we identify a region on chromosome V that is responsible for maintaining fertility at higher temperatures. Thus, we conclude that fitness of P. pacificus at high temperature is under genetic control, suggesting that it could be subject to natural selection.

Cells change their sensitivity to an EGF morphogen gradient to control EGF-induced gene expression.

How cells in developing organisms interpret the quantitative information contained in morphogen gradients is an open question. Here we address this question using a novel integrative approach that combines quantitative measurements of morphogen-induced gene expression at single-mRNA resolution with mathematical modelling of the induction process. We focus on the induction of Notch ligands by the LIN-3/EGF morphogen gradient during vulva induction in Caenorhabditis elegans. We show that LIN-3/EGF-induced Notch ligand expression is highly dynamic, exhibiting an abrupt transition from low to high expression. Similar transitions in Notch ligand expression are observed in two highly divergent wild C. elegans isolates. Mathematical modelling and experiments show that this transition is driven by a dynamic increase in the sensitivity of the induced cells to external LIN-3/EGF. Furthermore, this increase in sensitivity is independent of the presence of LIN-3/EGF. Our integrative approach might be useful to study induction by morphogen gradients in other systems.

Cryptic variation in vulva development by cis-regulatory evolution of a HAIRY-binding site.

Robustness to mutations is a general principle of biological systems that allows for the accumulation of cryptic variation. However, little is known about robustness and cryptic variation in core developmental pathways. Here we show through gonad-ablation screens in natural isolates of Pristionchus pacificus cryptic variation in nematode vulva development. This variation is mainly caused by cis-regulatory evolution in the conserved Notch ligand apx-1/Delta and involves binding sites for the transcription factor HAIRY. In some isolates, including a Bolivian strain, absence of a HAIRY-binding site results in Ppa-apx-1 expression in the vulva precursor cell P6.p and causes gonad-independent vulva differentiation. In contrast, a Californian strain that gained a HAIRY-binding site lacks Ppa-apx-1 vulval expression and shows gonad-dependence of vulva development. Addition of this HAIRY-binding site to the Bolivian Ppa-apx-1 promoter eliminates expression in the vulva. Our findings indicate significant cis-regulatory evolution in a core developmental pathway leading to intraspecific cryptic variation.