Pristionchus pacificus has been established as a nematode model system in evolutionary developmental biology and evolutionary ecology. Field studies in North and South America, Asia, Africa and Europe indicated that nematodes of the genus Pristionchus live in association with scarab beetles. Here, we describe the first account of soil-and beetle-associated nematodes on an island setting by investigating the island of Réunion in the Indian Ocean. Réunion has high numbers of endemic insects and is one among several attractive islands for biodiversity studies. Being of volcanic origin, Réunion is 2-3 million years old, making it the youngest of the Mascareigne islands. We show that beetle-and soil-derived nematodes on Réunion are nearly exclusively hermaphroditic, suggesting that selfing is favoured over gonochorism (outcrossing) during island colonization. Among members of four nematode genera observed on Réunion, Pristionchus pacificus was the most prevalent species. A total of 76 isolates, in association with five different scarab beetles, has been obtained for this cosmopolitan nematode. A detailed mitochondrial haplotype analysis indicates that the Réunion isolates of P. pacificus cover all four worldwide clades of the species. This extraordinary haplotype diversity suggests multiple independent invasions, most likely in association with different scarab beetles. Together, we establish Réunion as a case study for nematode island biogeography, in which the analysis of nematode population genetics and population dynamics can provide insight into evolutionary and ecological processes.
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.
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.
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.
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.
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