Thermal regime drives a latitudinal gradient in morphology and life history in a livebearing fish

Abstract

Within-species diversity is often driven by changing selective regimes along environmental gradients. Here, we provide a direct test of the environmental factors underlying phenotypic diversity across the wide native distribution of eastern mosquitofish (Gambusia holbrooki). We investigated life-history and body-shape divergence (including multiple measures of body size) across more than 14 degrees of latitude in North America, and used Akaike's information criterion-based model selection to determine the relative contributions of thermal regime, population densities and habitat productivity as potential drivers of latitudinal phenotypic variation. We found thermal regime to be the most important driver of large-scale latitudinal phenotypic patterns: populations in colder climates with greater seasonality and range in temperature exhibited larger body size, larger reproductive investment coupled with smaller offspring size, and shallower bodies with a smaller head and more anterodorsally positioned pectoral fins. Nonetheless, population density and habitat productivity also influenced trait divergence, but independent of latitudinal patterns, and some variation in body shape was due to apparent covariation with life histories. Our study confirms thermal regime as an important driver of latitudinal phenotypic differentiation even in ectotherms, but also uncovers multiple additional factors that shape phenotypic diversity, emphasizing the importance of the multivariate approach we employed here.ADDITIONAL KEYWORDS: body size -Bergmann's rule -Gambusia holbrooki -population densityproductivity -temperature gradient.

Geographical and temporal variation of multiple paternity in invasive mosquitofish (Gambusia holbrooki, Gambusia affinis)

“…At first sight, this result seems to provide support for direct climate‐effects, for instance, via female solicitation of MP to increase genetic variation and/or quality of their offspring under harsh (northern) conditions (Table ). However, path analysis suggested that climatic effects were mainly indirect, mediated by altered life‐history traits, both latitude‐dependent (Ouyang et al, ; Riesch et al, ) and ‐independent. Both types of analyses, however, received relatively low statistical support.…”

“…Northern populations are subject to higher overwinter mortality than southern ones (Cheng et al, ), leading to lower population densities and female‐biased sex ratios, at least at the beginning of the reproductive season (Zulian, Bisazza, & Marin, ). While mosquitofish males are, on average, larger at higher latitudes (Ouyang et al, ; Riesch et al, ), some males can achieve sexual maturity at particularly small body sizes under these conditions (Zulian et al, ; see also Borowsky, for Xiphophorus variatus ), and smaller males are more efficient in sneaky copulations, being more manoeuvrable and more difficult to spot by females (Pilastroet al, ).…”

“…Along latitudinal gradients, the amount of MP increased with latitude. In both the native and the invasive distribution ranges, northward expansions of Gambusia are mainly limited by low temperatures (Benejam et al, ; Riesch et al, ), while southern populations have often reached some geographical barrier that prevents further range expansion towards the South (the Mediterranean Ocean and Gulf of Mexico/Atlantic Ocean for G. holbrooki in their invasive and native range, respectively, and the South China Sea for G. affinis in China). Thus, the greatest potential for future range expansion is along the northern distribution edges in both invasive ranges, and it is exactly here that both species of Gambusia show the highest levels of MP.…”

“…We obtained population/monthly means of life‐history traits for both our geographical and temporal samples using well‐established protocols (Ouyang et al, ; Riesch, Easter, Layman, & Langerhans, ; Riesch et al, ). For the geographical sample, we used a subset of 19 populations (leaving out Baoding and Shenzhen for lack of mature/pregnant individuals).…”

“…A brief summary of these mechanisms-including mechanisms supporting our specific a priori hypotheses and additional explanations for variation in MP-are provided in Table 1. For example, number of sires per clutch usually increases as fecundity increases (Neff et al, 2008), and fecundity has been shown to increase towards northern populations in native G. holbrooki from the East Coast of the USA (Riesch et al, 2018). At the same time, increased male body size in northern populations could lead to a reduction of MP in those populations, as bigger males are able to monopolise access to females (Bisazza & Marin, 1991), and smaller males have higher efficiency in coercive copulation attempts due to their higher manoeuvrability (Pilastro, Giacomello, & Bisazza, 1997).…”