Morphogenesis of Paragordius varius (Nematomorpha) during the parasitic phase

Rampant loss of universal metazoan genes revealed by a chromosome-level genome assembly of the parasitic Nematomorpha

Parasites may manipulate host behavior to increase the odds of transmission or to reach the proper environment to complete their life cycle.1,2 Members of the phylum Nematomorpha (known as horsehair worms, hairworms, or Gordian worms) are large endoparasites that affect the behavior of their arthropod hosts. In terrestrial hosts, they cause erratic movements toward bodies of water,3,4,5,6 where the adult worm emerges from the host to find mates for reproduction. We present a chromosome-level genome assembly for the freshwater Acutogordius australiensis and a draft assembly for one of the few known marine species, Nectonema munidae. The assemblies span 201 Mbp and 213 Mbp in length (N50: 38 Mbp and 716 Kbp), respectively, and reveal four chromosomes in Acutogordius, which are largely rearranged compared to the inferred ancestral condition in animals. Both nematomorph genomes have a relatively low number of genes (11,114 and 8,717, respectively) and lack a high proportion (∼30%) of universal single-copy metazoan orthologs (BUSCO genes7). We demonstrate that missing genes are not an artifact of the assembly process, with the majority of missing orthologs being shared by the two independent assemblies. Missing BUSCOs are enriched for Gene Ontology (GO) terms associated with the organization of cilia and cell projections in other animals. We show that most cilium-related genes conserved across eukaryotes have been lost in Nematomorpha, providing a molecular basis for the suspected absence of ciliary structures in these animals.

Mitochondria in Obliquely Striated Muscles of the Horsehair Worm Gordionus alpestris (Nematomorpha, Gordioidea) with Structural Organization Typical of Cells with Energy-Intensive Processes

The ultrastructure of mitochondria in the flattened circomyarian fibers of the horsehair worm Gordionus alpestris (Nemathelminthes) was examined. In contrast to the previously published data, we showed these mitochondria to be giant elongated organelles that densely fill the central cytoplasmic space of the ribbon-like muscle fibers. No fundamental differences were found in the ultrastructure of the muscle tissue mitochondria in actively moving free-living and parasitic G. alpestris worms. The functional significance of the observed ultrastructural organization of mitochondria is discussed in connection with the necessity for an extended mitochondrial membrane system for a uniform supply of active muscle tissue with energy.

Going solo: discovery of the first parthenogenetic gordiid (Nematomorpha: Gordiida)

Despite the severe fitness costs associated with sexual reproduction, its persistence and pervasiveness among multicellular organisms testifies to its intrinsic, short-term advantages. However, the reproductive assurance hypothesis predicts selection favoring asexual reproduction in sparse populations and when mate finding is difficult. Difficulties in finding mates is especially common in parasites, whose life cycles involve multiple hosts, or being released from the host into the external environment where the parasite can find itself trapped without a sexual partner. To solve this problem and guarantee reproduction, parasites in numerous phyla have evolved reproductive strategies, as predicted by the reproductive assurance hypothesis, such as hermaphroditism or parthenogenesis. However, this type of strategy has not been reported from species in the phylum Nematomorpha, whose populations have often been described as sparse. A new Nematomorpha species, Paragordius obamai n. sp., was discovered from Kenya, Africa, and appears to have solved the problem of being trapped without a mate by eliminating the need for males. Paragordius obamai n. sp. represents the first and only known species within this phylum to reproduce asexually. To determine the mechanism of this mating strategy, we ruled out the involvement of reproduction manipulating endosymbionts by use of next generation sequencing data, thus suggesting that parthenogenesis is determined genetically and may have evolved as a means to assure reproduction. Since this new parthenogenetic species and a closely related gonochoristic North American congener, P. varius, are easy to propagate in the laboratory, these gordiids can be used as model systems to test hypotheses on the genetic advantages and disadvantages of asexual reproduction and the genetic determinants of reproductive strategies in parasites.

Biology of the phylum nematomorpha

Compared with most animal phyla, the Nematomorpha, also known as hair worms, is a relatively understudied metazoan phylum. Although nematomorphs make up only 1 of 3 animal phyla specializing solely on a parasitic life style, little attention has been focused on this enigmatic group scientifically. The phylum contains two main groups. The nectonematids are parasites of marine invertebrates such as hermit crabs. The gordiids are parasites of terrestrial arthropods, such as mantids, beetles, and crickets. Members of both of these groups are free-living as adults in marine and freshwaters respectively. In recent years, large strides have been made to understand this group more fully. New information has come from collection efforts, new approaches in organismal biology, modern techniques in microscopy and molecular biology. This review will focus on the advances made in four main areas of research: (1) morphology, (2) taxonomy and systematics, (3) life cycle and ecology and (4) host behavioural alterations. Recent research focus on the structure of both nectonematids and gordiids has added new insights on the morphology of adult worms and juveniles. The nervous system of gordiids is now well described, including the documentation of sensory cells. In addition, the availability of material from the juvenile of several species of gordiids has made it possible to document the development of the parasitic stage. New collections and reinvestigations of museum specimens have allowed for a critical reevaluation of the validity of established genera and species. However, traditional taxonomic work on this group continues to be hampered by two impeding factors: first is the lack of species-specific characters; and second is the problem of intraspecific variation, which has likely led to the description of numerous synonyms. Modern molecular techniques have been used recently to support independently the broad relationships among gordiids. During the turn of the millennium, the study of the life cycle and general ecology of gordiids enjoyed a revival. The pivotal outcome of this research was the domestication of a common American gordiid species, Paragordius varius. This species was the first of this phylum to be laboratory-reared. Through this research, the life cycle of several distantly related gordiid species was investigated. Other work showed that gordiids persist in the environment in the cyst stage by moving through different hosts by paratenesis. These cysts have been shown to retain infectivity for up to a year. These factors have likely contributed to the finding that gordiid cysts are one of the most common metazoans in some aquatic environments. Finally, recent work has focused on elucidating the mechanism of how gordiids make the transition from terrestrially based definitive hosts to a free-living aquatic environment. It has been shown that hosts are manipulated by the parasites to enter water. Using this study system, and using histology and proteomic tools, the method of manipulation used by these parasites is being further investigated. This manipulation, and the reaction of the cricket to this manipulation, has been postulated to benefit both the parasite and the host. Although large strides have been made within the last 10 years in the understanding of nematomorphs, we make the case that a lot of basic information remains to be uncovered. Although seemingly a daunting task, the recent advances in information and techniques lay a solid foundation for the future study of this unique group of parasites.

An anomaly against a current paradigm--extremely low rates of individual fecundity variability of the Gordian worm (Nematomorpha: Gordiida)

SUMMARY

Extreme variation in reproductive success (VRS) has been reported as a common feature of populations. Few individuals producing most of the offspring for the next generation has potential consequences for the population dynamics, genetics, and evolution of a group of organisms. High VRS has been described as a normal feature of helminth populations, although studies have focused largely on parasites of vertebrate hosts. Paragordius varius, a parasite of crickets, was used as a model system to study VRS. In this life cycle, worms absorb and store resources for reproduction from their hosts before being released into water. Egg output varied significantly with worm length, indicating that female length is an excellent predictor of fecundity. Analyses using the Lorenz curve and Gini coefficient suggest that there were no marked fecundity differences. This result was supported by data collected from a natural gordiid population, Gordius difficilis, suggesting that within gordiid populations the offspring of the next generation are contributed nearly equally by females. In addition, male body length appeared to be limited by intensity, whereas females showed no length limitation by crowding. These results contrast previous studies of parasites.