The Ecology of Freshwater Planarians

Planarians are on the rise as a model system for regeneration and stem cell dynamics. Almost in parallel the interest in planarian field biology has declined. Besides representing an independent research discipline in its own right, understanding of the natural habitat is also directly relevant to optimizing culture conditions in the laboratory. Moreover, the current laboratory models are but few of hundreds of planarian species worldwide. Their adaptation to a wide range of ecological niches has resulted in a fascinating diversity of regenerative abilities, body size, reproduction strategies, and life expectancy, to name just a few. With the currently ongoing establishment of large planarian species collections, such phenotypic diversity becomes accessible to comparative mechanistic analysis in the laboratory. Overall, we hope that this chapter inspires an integral view of the planarian model system that not only includes the molecular and cellular processes under investigation but also the evolutionary forces that shaped them in the first place.

Exploring the sexual diversity of flatworms: Ecology, evolution, and the molecular biology of reproduction

Flatworms exhibit huge diversity in their reproductive biology, making this group an excellent model system for exploring how differences among species in reproductive ecology are reflected in the physiological and molecular details of how reproduction is achieved. In this review, I consider five key "lifestyle choices" (i.e., alternative evolutionary/developmental outcomes) that collectively encompass much of flatworm sexual diversity, beginning with the decisions: (i) whether to be free-living or parasitic; (ii) whether to reproduce asexually or sexually; and (iii) whether to be gonochoristic (separate-sexed) or hermaphroditic. I then examine two further decisions involving hermaphroditism: (iv) outcrossing versus selfing and (v) the balance of investment into the male versus the female sex function (sex allocation). Collectively, these lifestyle choices set the basic rules for how reproduction occurs, but as I emphasize in the second part of the review, the reproductive biology of flatworms is also greatly impacted by the near-pervasive and powerful pressure of sexual selection, together with the related phenomena of sperm competition and sexual conflict. Exactly how this plays out, however, is strongly affected by the particular combination of reproductive strategies adopted by each species. Mol. Reprod. Dev. 84: 120-131, 2017. © 2016 Wiley Periodicals, Inc.

Thelytokous parthenogenesis in the damselfly Ischnura hastata (Odonata, Coenagrionidae): genetic mechanisms and lack of bacterial infection

Thelytokous parthenogenesis, the production of female-only offspring from unfertilized eggs, has been described in all the insect orders, but is a rare phenomenon in the Odonata (dragonflies and damselflies). The only-known case of parthenogenesis in this group is the North American damselfly species Ischnura hastata, which has parthenogenetic populations in the Azores Islands. Here, we present for the first time the results of laboratory rearing, which showed parthenogenetic reproduction in the Azorean I. hastata populations. In an attempt to understand how parthenogenesis could have evolved in this species, we first determined the genetic mode of parthenogenesis by analysing the genotype of parthenogenetic females and their offspring at three polymorphic microsatellite loci. In addition, we used polymerase chain reaction amplification to test whether parthenogenesis in I. hastata could be bacterially induced. Our data indicate that thelytoky is achieved through an (at least functionally) apomictic mechanism and that parthenogenesis is not caused by endosymbionts. Finally, we discuss possible routes to parthenogenetic reproduction, as well as the evolutionary implications of this type of parthenogenesis.

Paternal inheritance in parthenogenetic forms of the planarian Schmidtea polychroa

Parthenogenesis usually includes clonal inheritance, which is thought to increase the risk of the clonal populations' extinction. Yet many parthenogenetic organisms appear to have survived for extended periods. A possible explanation is that parthenogens occasionally reproduce through sex-like processes. Although there is indirect evidence for occasional sex, the underlying mechanisms are currently unknown. In the present study, we examined sex-like processes in the planarian flatworm Schmidtea (Dugesia) polychroa. Parthenogenetic forms of this species are simultaneous hermaphrodites that require sperm to trigger embryogenesis, whereas paternal genetic material is usually excluded from the oocyte (sperm-dependent parthenogenesis). Based on a comparison of parents and offspring, using highly polymorphic microsatellites, we demonstrate the incorporation of paternal alleles in about 5% of the offspring. We detected two distinct processes: chromosome addition and chromosome displacement. Such rare sexual processes may explain the long-term persistence of the many purely parthenogenetic populations of S. polychroa in northern Europe.

Admixed sexual and facultatively asexual aphid lineages at mating sites

Cyclically parthenogenetic organisms may have facultative asexual counterparts. Such organisms, including aphids, are therefore interesting models for the study of ecological and genetic interactions between lineages differing in reproductive mode. Earlier studies on aphids have revealed major differences in the genetic outcomes of populations that are possibly resulting mostly either from sexual or from asexual reproduction. Besides, notable gene flow between sexual and asexual derivatives has been suspected, which could lead to the emergence of new asexual lineages. The present study examines the interplay between these lineages and is based on analyses of population structure of individuals that may contribute to the pool of sexual reproductive forms in the host alternating aphid Rhopalosiphum padi. Using a Bayesian assignment method, we first show that the sexual forms of R. padi on mating sites encompass two genetically distinct clusters of individuals in the western part of France. The first cluster included unique genotypes of sexual lineages, while the second cluster included facultatively asexual lineages in numerous copies, the reproductive mode of the two clusters being confirmed by reference clones. Sexual reproductive forms produced by sexual and facultatively asexual lineages are thus admixed at mating sites which gives a large opportunity for the two clusters to mate with each other. Nevertheless, this study also highlights, as previously demonstrated, that the two clusters retained high genetic differentiation. Possible explanations for the inferred limited genetic exchanges are advanced in the discussion, but further dedicated investigations are required to solve this paradox.

Occasional sex in an 'asexual' polyploid hermaphrodite

Asexual populations are usually considered evolutionary dead-ends because they lack the mechanisms to generate and maintain sufficient genetic diversity. Yet, some asexual forms are remarkably widespread and genetically diverse. This raises the question whether asexual systems are always truly clonal or whether they have cryptic forms of sexuality that enhance their viability. In the planarian flatworm Schmidtea polychroa parthenogens are functional hermaphrodites (as are their sexual conspecifics), copulate and exchange sperm. Sperm is required for initiation of embryogenesis but usually does not contribute genetically to the offspring (sperm-dependent parthenogenesis). Using karyology and genotyping of parents and offspring, we show that in a purely parthenogenetic population an estimated 12% of all offspring are the result of partial genetic exchange. Several processes of chromosome addition and loss are involved. Some of these result in an alternation between a common triploid and a rare tetraploid state. We conclude that genetic recombination does not necessarily require segregation and fusion within the same generation, as is the case in most sexual species. These occasional sexual processes help to explain the geographical dominance of parthenogens in our study species.

High multiple paternity and low last-male sperm precedence in a hermaphroditic planarian flatworm: consequences for reciprocity patterns

It is difficult to predict a priori how mating success translates into fertilization success in simultaneous hermaphrodites with internal fertilization. Whereas insemination decisions will be determined by male interests, fertilization will depend on female interests, possibly leading to discrepancies between insemination and fertilization patterns. The planarian flatworm Schmidtea polychroa, a simultaneous hermaphrodite in which mating partners trade sperm was studied. Sperm can be stored for months yet individuals mate frequently. Using microsatellites, maternity and paternity data were obtained from 748 offspring produced in six groups of 10 individuals during four weeks. Adults produced young from four mates on average. Reciprocal fertilization between two mates was found in only 41 out of 110 registered mate combinations, which is clearly less than what is predicted from insemination patterns. Multiple paternity was high: > 80% of all cocoons had two to five fathers for only three to five offspring per cocoon. Because animals were collected from a natural population, 28% of all hatchlings were sired by unknown sperm donors in the field, despite a 10-day period of acclimatization and within-group mating. This percentage decreased only moderately throughout the experiment, showing that sperm can be stored and used for at least a month, despite frequent mating and sperm digestion. The immediate paternity a sperm donor could expect to obtain was only about 25%. Male reproductive success increased linearly with the number of female partners, providing support for Bateman's principle in hermaphrodites. Our results suggest that hermaphrodites do not trade fertilizations when trading sperm during insemination, lending support to the view that such conditional sperm exchange is driven by exchange of resources.

Gene flow between arrhenotokous and thelytokous populations of Venturia canescens (Hymenoptera)

In the solitary parasitoid wasp Venturia canescens both arrhenotokously (sexual) and thelytokously (parthenogenetical) reproducing individuals occur sympatrically. We found in the laboratory that thelytokous wasps are able to mate, receive and use sperm of arrhenotokous males. Using nuclear (amplified fragment length polymorphism, virus-like protein) and mitochondrial (restriction fragment length polymorphism) markers, we show the occurrence of gene flow from the arrhenotokous to the thelytokous mode in the field. Our results reinforce the paradox of sex in this species.