Patterns of trait divergence between populations of the meadow grasshopper, Chorthippus parallelus

Abstract.— To understand the process of speciation, we need to identify the evolutionary phenomena associated with divergence between populations of the same species. A powerful approach is to compare patterns of trait differences between populations differing in their evolutionary histories. A recent study of genetic divergence between populations of the meadow grasshopper Chorthippus parallelus , from different locations around Europe has allowed us to use this species to investigate which aspects of evolutionary history are associated with divergence in morphology and mating signals. During the last glaciation C. parallelus was confined to a number of refugia in southern Europe and has subsequently recolonized the northern part of the continent. This process of isolation followed by range expansion has created populations differing markedly in their evolutionary pasts–some have been isolated from one another for thousands of years, others have undergone repeated founder events, and others now live in sympatry with a closely related species. Using laboratory-reared grasshoppers from 12 different populations with a range of evolutionary histories, we quantify differences in morphology, chemical signals, and male calling-song. The observed pattern of divergence between these populations is then compared with the pattern predicted by hypotheses about what drives divergence. This comparison reveals that long periods in allopatry and processes associated with repeated founder events are both strongly associated with divergence.     

EVALUATION OF ELEVATED PLOIDY AND ASEXUAL REPRODUCTION AS ALTERNATIVE EXPLANATIONS FOR GEOGRAPHIC PARTHENOGENESIS IN EUCYPRIS VIRENS OSTRACODS

Transitions from sexual to asexual reproduction are often coupled with elevations in ploidy. As a consequence, the importance of ploidy per se for the maintenance and spread of asexual populations is unclear. To examine the effects of ploidy and asexual reproduction as independent determinants of the success of asexual lineages, we sampled diploid sexual, diploid asexual, and triploid asexual Eucypris virens ostracods across a European wide range. Applying nuclear and mitochondrial markers, we found that E. virens consists of genetically highly differentiated diploid sexual populations, to the extent that these sexual clades could be considered as cryptic species. All sexual populations were found in southern Europe and North Africa and we found that both diploid asexual and triploid asexual lineages have originated multiple times from several sexual lineages. Therefore, the asexual lineages show a wide variety of genetic backgrounds and very strong population genetic structure across the wide geographic range. Finally, we found that triploid, but not diploid, asexual clones dominate habitats in northern Europe. The limited distribution of diploid asexual lineages, despite their shared ancestry with triploid asexual lineages, strongly suggests that the wider geographic distribution of triploids is due to elevated ploidy rather than to asexuality.     

A metabarcoding analysis of the wrackbed microbiome indicates a phylogeographic break along the North Sea–Baltic Sea transition zone

Sandy beaches are biogeochemical hotspots that bridge marine and terrestrial ecosystems via the transfer of organic matter, such as seaweed (termed wrack). A keystone of this unique ecosystem is the microbial community, which helps to degrade wrack and re-mineralize nutrients. However, little is known about this community. Here, we characterize the wrackbed microbiome as well as the microbiome of a primary consumer, the seaweed fly Coelopa frigida, and examine how they change along one of the most studied ecological gradients in the world, the transition from the marine North Sea to the brackish Baltic Sea. We found that polysaccharide degraders dominated both microbiomes, but there were still consistent differences between wrackbed and fly samples. Furthermore, we observed a shift in both microbial communities and functionality between the North and Baltic Sea driven by changes in the frequency of different groups of known polysaccharide degraders. We hypothesize that microbes were selected for their abilities to degrade different polysaccharides corresponding to a shift in polysaccharide content in the different seaweed communities. Our results reveal the complexities of both the wrackbed microbial community, with different groups specialized to different roles, and the cascading trophic consequences of shifts in the near shore algal community.     

The origins of premating reproductive isolation: testing hypotheses in the grasshopper Chorthippus parallelus

There are many proposed routes for the origin of premating reproductive isolation, but few systematic studies aimed at testing their relative importance. Accumulated information about the biogeographical history of the European meadow grasshopper, Chorthippus parallelus, has allowed us to make a planned series of comparisons among populations aimed at distinguishing the contributions of some of these hypotheses. We have compared the effects on assortative mating of long-term isolation in glacial refugia, founder events during postglacial colonization, and sympatry with a closely related species. A likelihood-based analysis allowed us to separate effects of variation in male and female mating propensity among populations from variation in mate choice leading to assortative mating. All three effects contributed significantly to the overall variation in mating pattern in a set of 21 pairwise comparisons among seven populations. Male cuticular composition, but not other candidate signals, was significantly associated with the level of assortative mating. Of the hypotheses for the origin of reproductive isolation, only the predictions of the founder hypothesis explained a significant amount of the variation in assortative mating. This does not rule out the possiblity that there may be some other explanation. Having established the pattern of divergence, it is possible to generate hypotheses that explain our results at least as well as the founder hypothesis. However, because many such post hoc hypotheses are possible, they cannot be tested with this dataset. On this basis, our results favor the hypothesis that some aspect of the colonization process tends to accelerate divergence in mating signals leading to premating reproductive isolation. This could be accomplished through any one of several mechanisms. Colonization involves many bottlenecks as new populations are established at the edge of the range by long-distance migrants. Genetic effects may be important, but these bottlenecks may also alter the conditions under which mates are found and chosen, as suggested by Kaneshiro. At the same time, the colonizing populations may encounter novel environmental challenges.     

Inheritance of song and stridulatory peg number divergence between Chorthippus brunneus and C. jacobsi, two naturally hybridizing grasshopper species (Orthoptera: Acrididae)

Knowledge of the genetic basis of divergence in mating signal characters that contribute to reproductive isolation is critical to understanding speciation. Here, we describe a semi-automated system for characterizing grasshopper acoustic signals. We used this system to study the genetic basis of divergence in three male calling song components [echeme (EL), syllable (SL) and phrase (PL) lengths] between Chorthippus brunneus and C. jacobsi, two species of grasshoppers that hybridize in northern Spain. We also studied the number of pegs in the stridulatory file. For all characters, additive effects accounted for most of the genetic differentiation between species. However, the three song components also showed small but significant epistatic effects. No sex linkage was detected. Wright-Castle-Lande estimates of the minimum numbers of genetic factors underlying song and peg number divergence were low: peg number (n(e)=5.87+/-5.84), SL (n(e)=2.37+/-4.79) and PL (n(e)=0.87+/-0.86). On the other hand, EL appeared to be controlled by many genes. These results suggest that divergence in SL and PL might be driven by sexual selection whereas EL might not be under selection. This is consistent with experimental results on female song preference in related species. However, the fact that few factors appear to underlie the differences in peg number is surprising. Peg number is not closely related to song characteristics. It often varies between closely related grasshopper species and it has been assumed to be a neutral character. The biometrical approaches used here tend to underestimate the number of factors influencing a trait but provide valuable background for subsequent quantitative trait loci analyses.     

Recombination and speciation

Speciation can be viewed as the evolution of restrictions on the freedom of genetic recombination: new combinations of alleles can be generated within species, but alleles from different species cannot be brought together. Recently, there has been increasing realization that the role of chromosomal rearrangements in speciation might be primarily a result of their influence on recombination. I argue that ideas about the role of recombination in speciation should be considered in the context of the variability of recombination rates and patterns more generally and that genic as well as chromosomal causes of restricted recombination should be considered. I review patterns of variation in recombination rates and theoretical progress in understanding the conditions that favour increased or decreased rates. Although progress has been made in understanding conditions that alter overall rates of recombination, widespread variation in patterns of recombination remains largely unexplained. I consider three models for the role of locally restricted recombination in speciation and the evidence currently supporting them.     

Sexual selection and speciation

The power of sexual selection to drive changes in mate recognition traits gives it the potential to be a potent force in speciation. Much of the evidence to support this possibility comes from comparative studies that examine differences in the number of species between clades that apparently differ in the intensity of sexual selection. We argue that more detailed studies are needed, examining extinction rates and other sources of variation in species richness. Typically, investigations of extant natural populations have been too indirect to convincingly conclude speciation by sexual selection. Recent empirical work, however, is beginning to take a more direct approach and rule out confounding variables.