Assortative mating but no evidence of genetic divergence in a species characterized by a trophic polymorphism

Lack of host specialization despite selective host use in brood parasitic cuckoo catfish

Host-parasite dynamics involve coevolutionary arms races, which may lead to host specialization and ensuing diversification. Our general understanding of the evolution of host specialization in brood parasites is compromised by a restricted focus on bird and insect lineages. The cuckoo catfish (Synodontis multipunctatus) is an obligate parasite of parental care of mouthbrooding cichlids in Lake Tanganyika. Given the ecological and taxonomic diversity of mouthbrooding cichlids in the lake, we hypothesized the existence of sympatric host-specific lineages in the cuckoo catfish. In a sample of 779 broods from 20 cichlid species, we found four species parasitized by cuckoo catfish (with prevalence of parasitism of 2%-18%). All parasitized cichlids were from the tribe Tropheini, maternal mouthbrooders that spawn over a substrate (rather than in open water). Phylogenetic analysis based on genomic (ddRAD sequencing) and mitochondrial (Dloop) data from cuckoo catfish embryos showed an absence of host-specific lineages. This was corroborated by analyses of genetic structure and co-ancestry matrix. Within host species, parasitism was not associated with any individual characteristic we recorded (parent size, water depth), but was costly as parasitized parents carried smaller clutches of their own offspring. We conclude that the cuckoo catfish is an intermediate generalist and discuss costs, benefits and constraints of host specialization in this species and brood parasites in general.

Evolutionary divergence in phenotypic plasticity shapes brain size variation between coexisting sunfish ecotypes

Mechanisms that generate brain size variation and the consequences of such variation on ecological performance are poorly understood in most natural animal populations. We use a reciprocal-transplant common garden experiment and foraging performance trials to test for brain size plasticity and the functional consequences of brain size variation in Pumpkinseed sunfish (Lepomis gibbosus) ecotypes that have diverged between nearshore littoral and offshore pelagic lake habitats. Different age-classes of wild-caught juveniles from both habitats were exposed for 6 months to treatments that mimicked littoral and pelagic foraging. Plastic responses in oral jaw size suggested that treatments mimicked natural habitat-specific foraging conditions. Plastic brain size responses to foraging manipulations differed between ecotypes, as only pelagic sourced fish showed brain size plasticity. Only pelagic juveniles under 1 year-old expressed this plastic response, suggesting that plastic brain size responses decline with age and so may be irreversible. Finally, larger brain size was associated with enhanced foraging performance on live benthic but not pelagic prey, providing the first experimental evidence of a relationship between brain size and prey-specific foraging performance in fishes. The recent post-glacial origin of these ecotypes suggests that brain size plasticity can rapidly evolve and diverge in fish under contrasting ecological conditions.

Climate mediates color morph turnover in a species exhibiting alternative reproductive strategies

Sexual selection is considered the primary driver of morph turnover in many color polymorphic taxa, yet the potential for other factors (like climate) to contribute to polymorphism maintenance and evolution remains unclear. Appreciation for a role of environmental conditions in the maintenance and evolution of color polymorphisms has grown in recent years, generating evidence suggesting that color morphs linked to sexual selection may also diverge in climate sensitivity. Focusing on the three color components contributing to the male tree lizard (Urosaurus ornatus) color morphs, I reveal a marked concordance between patterns of turnover over space and time, with a general affinity of orange- and yellow-colored males to hotter, more variable conditions, and blue colored males to wetter, cooler conditions. An assessment of long-term turnover in the blue color component in response to recent climate change over the past 60 years reinforces these findings. Overall, behavioral asymmetries attributed to sexual selection likely expose competing morphs to divergent environmental conditions in heterogeneous habitats, creating opportunity for natural selection to shape climate sensitivities that also drive turnover in morph color composition. Ultimately, these processes may favor stark asymmetries in morph persistence over the coming decades.

Diet-based assortative mating through sexual imprinting

Speciation is facilitated by "magic traits," where divergent natural selection on such traits also results in assortative mating. In animal populations, diet has the potential to act as a magic trait if populations diverge in consumed food that incidentally affects mating and therefore sexual isolation. While diet-based assortative mating has been observed in the laboratory and in natural populations, the mechanisms causing positive diet-based assortment remain largely unknown. Here, we experimentally created divergent diets in a sexually imprinting species of mouse, Peromyscus gossypinus (the cotton mouse), to test the hypothesis that sexual imprinting on diet could be a mechanism that generates rapid and significant sexual isolation. We provided breeding pairs with novel garlic- or orange-flavored water and assessed whether their offspring, exposed to these flavors in utero and in the nest before weaning, later preferred mates that consumed the same flavored water as their parents. While males showed no preference, females preferred males of their parental diet, which is predicted to yield moderate sexual isolation. Thus, our experiment demonstrates the potential for sexual imprinting on dietary cues learned in utero and/or postnatally to facilitate reproductive isolation and potentially speciation.

OPEN RESEARCH BADGES

This article has earned an Open Data Badge for making publicly available the digitally-shareable data necessary to reproduce the reported results. The data is available at https://doi.org/10.5061/dryad.n1qq6v3.

A way forward with eco evo devo: an extended theory of resource polymorphism with postglacial fishes as model systems

A major goal of evolutionary science is to understand how biological diversity is generated and altered. Despite considerable advances, we still have limited insight into how phenotypic variation arises and is sorted by natural selection. Here we argue that an integrated view, which merges ecology, evolution and developmental biology (eco evo devo) on an equal footing, is needed to understand the multifaceted role of the environment in simultaneously determining the development of the phenotype and the nature of the selective environment, and how organisms in turn affect the environment through eco evo and eco devo feedbacks. To illustrate the usefulness of an integrated eco evo devo perspective, we connect it with the theory of resource polymorphism (i.e. the phenotypic and genetic diversification that occurs in response to variation in available resources). In so doing, we highlight fishes from recently glaciated freshwater systems as exceptionally well-suited model systems for testing predictions of an eco evo devo framework in studies of diversification. Studies on these fishes show that intraspecific diversity can evolve rapidly, and that this process is jointly facilitated by (i) the availability of diverse environments promoting divergent natural selection; (ii) dynamic developmental processes sensitive to environmental and genetic signals; and (iii) eco evo and eco devo feedbacks influencing the selective and developmental environments of the phenotype. We highlight empirical examples and present a conceptual model for the generation of resource polymorphism - emphasizing eco evo devo, and identify current gaps in knowledge.

Intraspecific brain size variation between coexisting sunfish ecotypes

Variation in spatial complexity and foraging requirements between habitats can impose different cognitive demands on animals that may influence brain size. However, the relationship between ecologically related cognitive performance and brain size is not well established. We test whether variation in relative brain size and brain region size is associated with habitat use within a population of pumpkinseed sunfish composed of different ecotypes that inhabit either the structurally complex shoreline littoral habitat or simpler open-water pelagic habitat. Sunfish using the littoral habitat have on average 8.3% larger brains than those using the pelagic habitat. We found little difference in the proportional sizes of five brain regions between ecotypes. The results suggest that cognitive demands on sunfish may be reduced in the pelagic habitat given no habitat-specific differences in body condition. They also suggest that either a short divergence time or physiological processes may constrain changes to concerted, global modifications of brain size between sunfish ecotypes.

Flexible mate choice may contribute to ecotype assortative mating in pumpkinseed sunfish (Lepomis gibbosus)

Gene flow is expected to limit adaptive divergence, but the ecological and behavioural factors that govern gene flow are still poorly understood, particularly at the earliest stages of population divergence. Reduced gene flow through mate choice (sexual isolation) can evolve even under conditions of subtle population divergence if intermediate phenotypes have reduced fitness. We indirectly tested the hypothesis that mate choice has evolved between coexisting littoral and pelagic ecotypes of polyphenic pumpkinseed sunfish (Lepomis gibbosus) that have diverged in morphology and resource use and where intermediate phenotypes have reduced performance. We assessed the ecotype of nesting males and females using stable isotope estimates of diet and a divergent male morphological trait, oral jaw width. We found positive assortative mating between ecotypes in a common spawning habitat along exposed lake shorelines, but contrary to expectations, assortative mating was variably expressed between two sampling years. Although the factors that influence variable assortative mating remain unclear, our results are consistent with mate choice being expressed by ecotypes. Despite being variably expressed, mate choice will reduce gene flow between ecotypes and could contribute to further adaptive divergence depending on its frequency and strength in the population. Our findings add to a growing body of evidence indicating mate choice behaviour can be a plastic trait, an idea that should be more explicitly considered in empirical studies of mate choice as well as conceptual frameworks of mate choice evolution and adaptive divergence.

Incipient sympatric speciation in Midas cichlid fish from the youngest and one of the smallest crater lakes in Nicaragua due to differential use of the benthic and limnetic habitats?

Understanding how speciation can occur without geographic isolation remains a central objective in evolutionary biology. Generally, some form of disruptive selection and assortative mating are necessary for sympatric speciation to occur. Disruptive selection can arise from intraspecific competition for resources. If this competition leads to the differential use of habitats and variation in relevant traits is genetically determined, then assortative mating can be an automatic consequence (i.e., habitat isolation). In this study, we caught Midas cichlid fish from the limnetic (middle of the lake) and benthic (shore) habitats of Crater Lake Asososca Managua to test whether some of the necessary conditions for sympatric speciation due to intraspecific competition and habitat isolation are given. Lake As. Managua is very small (<900 m in diameter), extremely young (maximally 1245 years of age), and completely isolated. It is inhabited by, probably, only a single endemic species of Midas cichlids, Amphilophus tolteca. We found that fish from the limnetic habitat were more elongated than fish collected from the benthic habitat, as would be predicted from ecomorphological considerations. Stable isotope analyses confirmed that the former also exhibit a more limnetic lifestyle than the latter. Furthermore, split‐brood design experiments in the laboratory suggest that phenotypic plasticity is unlikely to explain much of the observed differences in body elongation that we observed in the field. Yet, neutral markers (microsatellites) did not reveal any genetic clustering in the population. Interestingly, demographic inferences based on RAD‐seq data suggest that the apparent lack of genetic differentiation at neutral markers could simply be due to a lack of time, as intraspecific competition may only have begun a few hundred generations ago.