The origin of asexual brine shrimps

Determining how and how often asexual lineages emerge within sexual species is central to our understanding of sex-asex transitions and the long-term maintenance of sex. Asexuality can arise "by transmission" from an existing asexual lineage to a new one, through different types of crosses. The occurrence of these crosses, cryptic sex, variation in ploidy and recombination within asexuals greatly complicates the study of sex-asex transitions, as they preclude the use of standard phylogenetic methods and genetic distance metrics. In this study we show how to overcome these challenges by developing new approaches to investigate the origin of the various asexual lineages of the brine shrimp Artemia parthenogenetica. We use a large sample of asexuals, including all known polyploids, and their sexual relatives. We combine flow cytometry with mitochondrial and nuclear DNA data. We develop new genetic distance measures and methods to compare various scenarios describing the origin of the different lineages. We find that all diploid and polyploid A. parthenogenetica likely arose within the last 80,000 years through successive and nested hybridization events that involved backcrosses with different sexual species. All A. parthenogenetica have the same common ancestor and therefore likely carry the same asexuality gene(s) and reproduce by automixis. These findings radically change our view of sex-asex transitions in this group, and show the importance of considering asexuality "by transmission" scenarios. The methods developed are applicable to many other asexual taxa.

How land-use intensity affects sexual and parthenogenetic oribatid mites in temperate forests and grasslands in Germany

Intensive land use has been shown to alter the composition and functioning of soil communities. Due to their low dispersal ability, oribatid mites are particularly vulnerable to land-use intensification and species which are not adjusted to management-related disturbances become less abundant. We investigated how different land-use parameters in forests and grasslands affect oribatid mite diversity and abundance, with a focus on: (1) species-level impacts, by classifying species as increasing (‘winners’) or decreasing (‘losers’) in abundance with higher land-use intensity, and (2) reproductive impact, by investigating whether sexual and parthenogenetic species react differently. We collected 32,542 adult oribatid mites in 60 forests and grasslands of known land-use intensity in two regions of Germany. Diversity and total abundance as well as the proportion of sexual species were higher in forests than in grasslands. Diversity declined with higher land-use intensity in forests, but increased with higher mowing and fertilization in grasslands. Depending on land-use parameter and region, abundance either declined or remained unaffected by increasing intensity. Gravidity was higher in sexual than in parthenogenetic species and sexuals had 1.6× more eggs per gravid female. Proportions of sexual species and gravid females decreased with land-use intensity in forests, but increased with mowing in grasslands. At the species level, 75% of sexuals and 87.5% of parthenogens were ‘losers’ of higher percentages of dead wood originating from management-related disturbances. Across land-use parameters and habitats, a similar proportion of sexual and parthenogenetic oribatid mite species were ‘losers’ of high land-use intensity. However, ‘winner’ species were more common among sexuals.

Capture and return of sexual genomes by hybridogenetic frogs provide clonal genome enrichment in a sexual species

Hybridogenesis is a reproductive tool for sexual parasitism. Hybridogenetic hybrids use gametes from their sexual host for their own reproduction, but sexual species gain no benefit from such matings as their genome is later eliminated. Here, we examine the presence of sexual parasitism in water frogs through crossing experiments and genome-wide data. We specifically focus on the famous Central-European populations where Pelophylax esculentus males (hybrids of P. ridibundus and P. lessonae) live with P. ridibundus. We identified a system where the hybrids commonly produce two types of clonal gametes (hybrid amphispermy). The haploid lessonae genome is clonally inherited from generation to generation and assures the maintenance of hybrids through a process, in which lessonae sperm fertilize P. ridibundus eggs. The haploid ridibundus genome in hybrids received from P. ridibundus a generation ago, is perpetuated as clonal ridibundus sperm and used to fertilize P. ridibundus eggs, yielding female P. ridibundus progeny. These results imply animal reproduction in which hybridogenetic taxa are not only sexual parasites, but also participate in the formation of a sexual taxon in a remarkable way. This occurs through a process by which sexual gametes are being captured, converted to clones, and returned to sexual populations in one generation.

Adaptive and non-adaptive plasticity in changing environments: implications for sexual species with different life history strategies

Populations adapt to novel environmental conditions by genetic changes or phenotypic plasticity. Plastic responses are generally faster and can buffer fitness losses under variable conditions. Plasticity is typically modelled as random noise and linear reaction norms that assume simple one-to-one genotype-phenotype maps and no limits to the phenotypic response. Most studies on plasticity have focused on its effect on population viability. However, it is not clear, whether the advantage of plasticity depends solely on environmental fluctuations or also on the genetic and demographic properties (life histories) of populations. Here we present an individual-based model and study the relative importance of adaptive and non-adaptive plasticity for populations of sexual species with different life histories experiencing directional stochastic climate change. Environmental fluctuations were simulated using differentially autocorrelated climatic stochasticity or noise color, and scenarios of directional climate change. Non-adaptive plasticity was simulated as a random environmental effect on trait development, while adaptive plasticity as a linear, logistic, or sinusoidal reaction norm. The last two imposed limits to the plastic response and emphasized flexible interactions of the genotype with the environment. Interestingly, this assumption led to (i) smaller phenotypic than genotypic variance in the population and the coexistence of polymorphisms, (ii) many-to-one genotype-phenotype map, and (iii) the maintenance of higher genetic variation – compared to linear reaction norms and genetic determinism – even when the population was exposed to a constant environment for several generations. Limits to plasticity led to genetic accommodation, when costs were negligible, and to the appearance of cryptic variation when limits were exceeded. We found that adaptive plasticity promoted population persistence under red noise stochasticity and was particularly important for life histories with low fecundity. Populations producing more offspring could cope with environmental fluctuations solely by genetic changes or random plasticity, unless environmental change was too fast.

Parthenogenesis

The dominant mode of reproduction in eucaryotes is sexual. This has been described as a paradox given that sex is much more costly than reproducing asexually, such as by parthenogenesis. In the Crustacea, parthenogenesis is commonly found in the Ostracoda and Branchiopoda (Artemia and Cladocera), and studies of these species have made important contributions to understanding the ecological and evolutionary relationship between sexual and asexual reproduction. With respect to parthenogenesis, researchers have explored its taxonomic distribution and phylogeny, origin and mode, ecological genetics, and genomic signatures. Parthenogenetic Crustacea include both diploid and polyploid clones that have originated multiple times from related sexual species but appear to have a relatively limited evolutionary lifespan. Darwinulid ostracods may be one exception, with no known sexual forms and possibly an example of ancient asexuality, although this is controversial. Most parthenogenetic crustacean groups appear to have a wider geographic distribution than related sexual species and are often found in marginal habitats associated with higher latitudes and altitudes. Such patterns of geographic parthenogenesis have yet to be fully explained, but could possibly be due to colonization and adaptation advantages of asexuality; further studies are required to eliminate polyploidy alone as an explanation. There are many examples of parthenogenetic ostracods, cladocerans, and Artemia showing high levels of genetic diversity likely due to recent multiple origins from related sexual species. Phylogenetic analyses support this explanation and for Artemia and Daphnia, cases have been documented for rare functional males produced by parthenogenetic females that can mate with sexual females as a mechanism for generating new clonal lineages. The diversity of asexual species, combined with prior ecological and genetic information, suggests that crustaceans will continue as important models for understanding parthenogenesis, particularly with the application of new genomic tools.

List of vascular plants endemic to Britain, Ireland and the Channel Islands 2020

A list of 804 plants endemic to Britain, Ireland and the Channel Islands is broken down by country. There are 659 taxa endemic to Britain, 20 to Ireland and three to the Channel Islands. There are 25 endemic sexual species and 26 sexual subspecies, the remainder are mostly critical apomictic taxa. Fifteen endemics (2%) are certainly or probably extinct in the wild.