Courtship behavior, nesting microhabitat, and assortative mating in sympatric stickleback species pairs

Discrete genetic modules underlie divergent reproductive strategies in three-spined stickleback

A central challenge in biology is to understand how complex behaviors evolve. Given their importance to fitness, complex behavioral traits often evolve as an integrated package, but it is unclear whether suites of traits evolve through a few pleiotropic genetic changes, each affecting many behaviors, or by accumulating several changes that, when combined, give rise to an entire package of correlated traits. Typically, three-spined stickleback exhibit paternal care, a behavior that characterizes the entire Gasterosteidae family. However, an unusual "white" three-spined stickleback ecotype exhibits a suite of traits associated with the evolutionary loss of paternal care. In the white ecotype, males disperse embryos from their nests rather than care for them, build loose nests, exhibit high rates of courtship, and are relatively small in body size. These behavioral differences are apparent in stickleback reared in a common garden environment, suggesting the differences have a heritable basis. In an F2 intercross, we show that these traits are genetically uncorrelated and map to different genomic regions, suggesting that components of the white reproductive strategy segregate independently and evolved through the addition of multiple genetic changes. These results contribute to the growing body of evidence that the behavioral diversity observed in nature may often evolve by accumulating and combining alleles, each with modular effects, and show that this principle applies to a suite of behavioral traits that together form an integrated and adaptive strategy.

Non-random mating behaviour between diverging littoral and pelagic three-spined sticklebacks in an invasive population from Upper Lake Constance

Adaptive divergence and increased genetic differentiation among populations can lead to reproductive isolation. In Lake Constance, Germany, a population of invasive three-spined stickleback (Gasterosteus aculeatus) is currently diverging into littoral and pelagic ecotypes, which both nest in the littoral zone. We hypothesized that assortative mating behaviour contributes to reproductive isolation between these ecotypes and performed a behavioural experiment in which females could choose between two nest-guarding males. Behaviour was recorded, and data on traits relevant to mate choice were collected. Both females of the same and different ecotypes were courted with equal vigour. However, there was a significant interaction effect of male and female ecotypes on the level of aggression in females. Littoral females were more aggressive towards pelagic males, and pelagic females were more aggressive towards littoral males. This indicates rejection of males of different ecotypes in spite of the fact that littoral males were larger, more intensely red-coloured and more aggressive than the pelagic males-all mating traits female sticklebacks generally select for. This study documents the emergence of behavioural barriers during early divergence in an invasive and rapidly diversifying stickleback population and discusses their putative role in facilitating reproductive isolation and adaptive radiation within this species.

Divergence in Reproductive Behaviors Is Associated with the Evolutionary Loss of Parental Care

AbstractThe mechanisms underlying the divergence of reproductive strategies between closely related species are still poorly understood. Additionally, it is unclear which selective factors drive the evolution of reproductive behavioral variation and how these traits coevolve, particularly during early divergence. To address these questions, we quantified behavioral differences in a recently diverged pair of Nova Scotian three-spined stickleback (Gasterosteus aculeatus) populations, which vary in parental care, with one population displaying paternal care and the other lacking this. We compared both populations, and a full reciprocal F1 hybrid cross, across four major reproductive stages: territoriality, nesting, courtship, and parenting. We identified significant divergence in a suite of heritable behaviors. Importantly, F1 hybrids exhibited a mix of behavioral patterns, some of which suggest sex linkage. This system offers fresh insights into the coevolutionary dynamics of reproductive behaviors during early divergence and offers support for the hypothesis that coevolutionary feedback between sexual selection and parental care can drive rapid evolution of reproductive strategies.

Behavioral plasticity can facilitate evolution in urban environments

Plasticity-led evolution is central to evolutionary theory. Although challenging to study in nature, this process may be particularly apparent in novel environments such as cities. We document abundant evidence of plastic behavioral changes in urban animals, including learning, contextual, developmental, and transgenerational plasticities. Using behavioral drive as a conceptual framework, our analysis of notable case studies suggests that plastic behaviors, such as altered habitat use, migration, diurnal and seasonal activity, and courtship, can have faciliatory and cascading effects on urban evolution via spatial, temporal, and mate-choice mechanisms. Our findings highlight (i) the need to incorporate behavioral plasticity more formally into urban evolutionary research and (ii) the opportunity provided by urban environments to study behavioral mechanisms of plasticity-led processes.

Does genetic differentiation underlie behavioral divergence in response to migration barriers in sticklebacks? A common garden experiment

Abstract

Water management measures in the 1970s in the Netherlands have produced a large number of “resident” populations of three-spined sticklebacks that are no longer able to migrate to the sea. This may be viewed as a replicated field experiment, allowing us to study how the resident populations are coping with human-induced barriers to migration. We have previously shown that residents are smaller, bolder, more exploratory, more active, and more aggressive and exhibited lower shoaling and lower migratory tendencies compared to their ancestral “migrant” counterparts. However, it is not clear if these differences in wild-caught residents and migrants reflect genetic differentiation, rather than different developmental conditions. To investigate this, we raised offspring of four crosses (migrant ♂ × migrant ♀, resident ♂ × resident ♀, migrant ♂ × resident ♀, resident ♂ × migrant ♀) under similar controlled conditions and tested for differences in morphology and behavior as adults. We found that lab-raised resident sticklebacks exhibited lower shoaling and migratory tendencies as compared to lab-raised migrants, retaining the differences in their wild-caught parents. This indicates genetic differentiation of these traits. For all other traits, the lab-raised sticklebacks of the various crosses did not differ significantly, suggesting that the earlier-found contrast between wild-caught fish reflects differences in their environment. Our study shows that barriers to migration can lead to rapid differentiation in behavioral tendencies over contemporary timescales (~ 50 generations) and that part of these differences reflects genetic differentiation.

Significance statement

Many organisms face changes to their habitats due to human activities. Much research is therefore dedicated to the question whether and how organisms are able to adapt to novel conditions. We address this question in three-spined sticklebacks, where water management measures cut off some populations, prohibiting their seasonal migration to the North Sea. In a previous study, we showed that wild-caught “resident” fish exhibited markedly different behavior than migrants. To disentangle whether these differences reflect genetic differentiation or differences in the conditions under which the wild-caught fish grew up, we conducted crosses, raising the F1 offspring under identical conditions. As their wild-caught parents, the F1 of resident × resident crosses exhibited lower migratory and shoaling tendencies than the F1 of migrant × migrant crosses, while the F1 of hybrid crosses were intermediate. This suggests that ~ 50 years of isolation are sufficient to induce behaviorally relevant genetic differentiation.