Karin Kiontke

Trends, stasis, and drift in the evolution of nematode vulva development.

BACKGROUND A surprising amount of developmental variation has been observed for otherwise highly conserved features, a phenomenon known as developmental system drift. Either stochastic processes (e.g., drift and absence of selection-independent constraints) or deterministic processes (e.g., selection or constraints) could be the predominate mechanism for the evolution of such variation. We tested whether evolutionary patterns of change were unbiased or biased, as predicted by the stochastic or deterministic hypotheses, respectively. As a model, we used the nematode vulva, a highly conserved, essential organ, the development of which has been intensively studied in the model systems Caenorhabditis elegans and Pristionchus pacificus. RESULTS For 51 rhabditid species, we analyzed more than 40 characteristics of vulva development, including cell fates, fate induction, cell competence, division patterns, morphogenesis, and related aspects of gonad development. We then defined individual characters and plotted their evolution on a phylogeny inferred for 65 species from three nuclear gene sequences. This taxon-dense phylogeny provides for the first time a highly resolved picture of rhabditid evolution and allows the reconstruction of the number and directionality of changes in the vulva development characters. We found an astonishing amount of variation and an even larger number of evolutionary changes, suggesting a high degree of homoplasy (convergences and reversals). Surprisingly, only two characters showed unbiased evolution. Evolution of all other characters was biased. CONCLUSIONS We propose that developmental evolution is primarily governed by selection and/or selection-independent constraints, not stochastic processes such as drift in unconstrained phenotypic space.

Evolution of early embryogenesis in rhabditid nematodes.

The cell-biological events that guide early-embryonic development occur with great precision within species but can be quite diverse across species. How these cellular processes evolve and which molecular components underlie evolutionary changes is poorly understood. To begin to address these questions, we systematically investigated early embryogenesis, from the one- to the four-cell embryo, in 34 nematode species related to C. elegans. We found 40 cell-biological characters that captured the phenotypic differences between these species. By tracing the evolutionary changes on a molecular phylogeny, we found that these characters evolved multiple times and independently of one another. Strikingly, all these phenotypes are mimicked by single-gene RNAi experiments in C. elegans. We use these comparisons to hypothesize the molecular mechanisms underlying the evolutionary changes. For example, we predict that a cell polarity module was altered during the evolution of the Protorhabditis group and show that PAR-1, a kinase localized asymmetrically in C. elegans early embryos, is symmetrically localized in the one-cell stage of Protorhabditis group species. Our genome-wide approach identifies candidate molecules-and thereby modules-associated with evolutionary changes in cell-biological phenotypes.

Description of Caenorhabditis japonica n. sp. (Nematoda: Rhabditida) associated with the burrower bug Parastrachia japonensis (Heteroptera: Cydnidae) in Japan

Caenorhabditis japonica n. sp. is described from Parastrachia japonensis from Japan. The species is closely related to species of the Caenorhabditis elegans group and shares many characters with them. It differs from these species in having blunt spicule tips of complex shape and in lacking a terminal notch in the bursa velum. Caenorhabditis japonica n. sp. is further characterised by an anterior end with the lips fused in pairs, long and pointed stegostomal teeth, long fringes on the anterior bursa margin and the form of the genital papillae (GP4 reduced). The species is integrated into the phylogenetic tree of Caenorhabditis. Some resulting consequences for character evolution within Caenorhabditis are discussed. Caenorhabditis japonica n. sp. is associated with a burrower bug, thereby adding a new component to the diverse ecology of Caenorhabditis species.

Phenotypic Plasticity: Different Teeth for Different Feasts

A polyphenism in the nematode Pristionchus pacificus involves the development of different feeding structures in response to an environmental cue, providing a genetic model species for investigating ecologically relevant phenotypic plasticity.

A critique of Rossberg et al.: noise obscures the genetic signal of meiobiotal ecospecies in ecogenomic datasets

High-throughput sequencing of DNA marker genes recovered from environmental samples (known as ecogenomics or metabarcoding) is an emerging tool for understanding patterns and processes in ecology and biodiversity [[1][1]]. The recent paper ‘Are there species smaller than 1 mm?’ [[2][2]] was

Description of Caenorhabditis angaria n. sp. (Nematoda: Rhabditidae), an associate of sugarcane and palm weevils (Coleoptera: Curculionidae)

Caenorhabditis angaria n. sp., an ectophoretic associate of the West Indian sugarcane weevil, Metamasius hemipterus (Coleoptera: Curculionidae) is described and illustrated. Data on biology (longevity, fecundity) and ecology are presented. Caenorhabditis angaria n. sp. is gonochoristic and can be differentiated from other species of Caenorhabditis by its comparatively short stoma in combination with six semicircular overlapping flaps on the lips, lack of a pharyngeal sleeve, one pair of teeth on each sector of the metastegostom, and a proximally open bursa with nine pairs of genital papillae (GP) and papilliform phasmids (ph) in a 2/2 + 2 + 3 + ph arrangement with GP4 and 7 opening dorsally. Caenorhabditis angaria n. sp. was isolated and cultured from M. hemipterus from Dade, Broward and Palm Beach Counties, FL, USA, and from Port-of-Spain, Trinidad, and from the American palm weevil, Rhynchophorus palmarum , from Trinidad. The nematode is phoretically associated with weevils as dauer juveniles without causing obvious deleterious effects. Caenorhabditis angaria n. sp. does not require the association with a weevil and can be cultured continuously on bacteria.

Redescription of Diplogasteroides nasuensis Takaki, 1941 and D. magnus völk, 1950 (Nematoda: Diplogastrina) associated with Scarabaeidae (Coleoptera)

A detailed morphological description is presented of Diplogasteroides nasuensis Takaki, 1941 and D. magnus Volk, 1950, using light and scanning electron microscopy. Neoaplectana melolonthae Weiser, 1958 and Diplogasteroides (Rhabdontolaimus) berwigi Ruhm, 1959 are synonymised with D. nasuensis. This gonochoristic species is distinguished from the hermaphroditic D. magnus by the dorsal metastomal decoration which consists of bristle-like protrusions instead of a more or less uniform tooth. The spermatocytes in D. nasuensis are twice as big as those in hermaphrodites of D. magnus. Dauer juveniles of D. nasuensis are considerably bigger than those of D. magnus. Dauer juveniles of both species are present on wood cockchafer (Melolontha hippocastani) larvae and adults but, on adult beetles, D. nasuensis dauer juveniles enter the genital pouch, whereas those of D. magnus are found under the hind coxae. In Germany, D. nasuensis is found only on M. hippocastani. D. magnus dauer juveniles are also found on a variety of other Scarabaeidae. Further small differences distinguish the two species. The genus Diplogasteroides is proposed to be taken in a broad sense. Several genera names are synonymised with Diplogasteroides.

Description of two three-gendered nematode species in the new genus Auanema (Rhabditina) that are models for reproductive mode evolution

The co-existence of males, females and hermaphrodites, a rare mating system known as trioecy, has been considered as an evolutionarily transient state. In nematodes, androdioecy (males/hermaphrodites) as found in Caenorhabditis elegans, is thought to have evolved from dioecy (males/females) through a trioecious intermediate. Thus, trioecious species are good models to understand the steps and requirements for the evolution of new mating systems. Here we describe two new species of nematodes with trioecy, Auanema rhodensis and A. freiburgensis. Along with molecular barcodes, we provide a detailed analysis of the morphology of these species, and document it with drawings and light and SEM micrographs. Based on morphological data, these free-living nematodes were assigned to a new genus, Auanema, together with three other species described previously. Auanema species display convergent evolution in some features with parasitic nematodes with complex life cycles, such as the production of few males after outcrossing and the obligatory development of dauers into self-propagating adults.

The association of two Diplogasteroides species (Secernentea: Diplogastrina) and cockchafers (Melolontha spp., Scarabaeidae)

The life cycle of two morphologically very similar Diplogasteroides species and their association with cockchafers in southern Germany was investigated. 70-100% of cockchafer grubs and 95% of the imagines carried Diplogasteroides spp. dauer juveniles. The nematodes were almost exclusively found on the external cuticle of the insects and usually not in the body cavity or the intestine. Diplogasteroides spp. dauer juveniles embark on the grub and accumulate during its development. There was some indication that dauer juveniles are transmitted from male to female beetle during copulation. The dauer juveniles resume development only after the death of the beetle, feeding on the cadaver (necromeny). Former hypotheses, assuming the nematode species to be parasitic and to cause the death of cockchafer grubs, can be refuted.

Makorin ortholog LEP-2 regulates LIN-28 stability to promote the juvenile-to-adult transition in Caenorhabditis elegans

The heterochronic genes lin-28, let-7 and lin-41 regulate fundamental developmental transitions in animals, such as stemness versus differentiation and juvenile versus adult states. We identify a new heterochronic gene, lep-2, in Caenorhabditis elegans. Mutations in lep-2 cause a delay in the juvenile-to-adult transition, with adult males retaining pointed, juvenile tail tips, and displaying defective sexual behaviors. In both sexes, lep-2 mutants fail to cease molting or produce an adult cuticle. We find that LEP-2 post-translationally regulates LIN-28 by promoting LIN-28 protein degradation. lep-2 encodes the sole C. elegans ortholog of the Makorin (Mkrn) family of proteins. Like lin-28 and other heterochronic pathway members, vertebrate Mkrns are involved in developmental switches, including the timing of pubertal onset in humans. Based on shared roles, conservation and the interaction between lep-2 and lin-28 shown here, we propose that Mkrns, together with other heterochronic genes, constitute an evolutionarily ancient conserved module regulating switches in development. Highlighted article: The C. elegans Makorin homolog lep-2 is a heterochronic gene required for the proper timing of the juvenile-to-adult transition. It acts in the LIN-28/let-7 pathway.