Evolution of a sexually dimorphic trait in a broadly distributed topminnow (Fundulus olivaceus)

Abstract

Understanding the interaction between sexual and natural selection within variable environments is crucial to our understanding of evolutionary processes. The handicap principle predicts females will prefer males with exaggerated traits provided those traits are indicators of male quality to ensure direct or indirect female benefits. Spatial variability in ecological factors is expected to alter the balance between sexual and natural selection that defines the evolution of such traits. Male and female blackspotted topminnows (Fundulidae: Fundulus olivaceus) display prominent black dorsolateral spots that are variable in number across its broad range. We investigated variability in spot phenotypes at 117 sites across 13 river systems and asked if the trait was sexually dimorphic and positively correlated with measures of fitness (condition and gonadosomatic index [GSI]). Laboratory and mesocosm experiments assessed female mate choice and predation pressure on spot phenotypes. Environmental and community data collected at sampling locations were used to assess predictive models of spot density at the individual, site, and river system level. Greater number of spots was positively correlated with measures of fitness in males. Males with more spots were preferred by females and suffered greater mortality due to predation. Water clarity (turbidity) was the best predictor of spot density on the drainage scale, indicating that sexual and natural selection for the trait may be mediated by local light environments.

Scale dependence of sex‐specific movement in a small‐bodied stream fish

“…For example, body condition of male F . olivaceus is a predictor of reproductive investment (Schaefer et al., ) and was positively related to male movement magnitude in this study. While it is important to note that our levels of density (high and low) were not necessarily fixed through time as some mortality probably occurred throughout the study, this would have enhanced any density effects.…”

“…For instance, male F . olivaceus are larger than females and exhibit a suite of secondary sexual characteristics such as dorsolateral spots and nuptial coloration that are preferred by females (Schaefer et al., ). Male F .…”

“…Male F . olivaceus with higher dorsolateral spot densities sired a disproportionately greater number of offspring in mate choice trials, but also suffered increased predation pressure (Schaefer et al., ). Whether these secondary sexual characters are correlated with behavioural traits in F .…”

“…Fundulus olivaceus (blackspotted topminnow) is a broadly distributed, small-bodied stream fish (maximum reported size 97 mm total length; Braasch & Smith, 1965) that exhibits a promiscuous, female-choice mating system (Schaefer et al, 2012). Adults are sexually dimorphic (distinguished by median fin morphology), reaching reproductive maturity within 1 year at approximately 35-40 mm (Blanchard, 1996;Vigueira, Schaefer, Duvernell, & Kreiser, 2008) and a maximum lifespan of 2-3 years (Ross, 2001).…”

Isolation and characterization of microsatellite loci for the blackstripe topminnow Fundulus notatus and their variability in two closely related species

“…Furthermore, freshwater species are found in environments that provide naturally replicated systems as a result of stream connectivity and the underlying ecological gradients (Vannote et al ., ). For these reasons, the F. notatus species complex has proven to be a useful model system for a variety of questions such as reproductive biology (Vigueira et al ., ; Schaefer et al ., ), hybrid zone dynamics (Duvernell et al ., ; Schaefer et al ., , ; Duvernell & Schaefer, ), physiology (Schaefer & Walters, ; Schaefer, ), evolutionary ecology (Alldredge et al ., ; Schaefer et al ., ) and phylogeography (Duvernell et al ., ). Molecular tools applied in the course of some of these studies have included sequencing and restriction enzyme digests of both mitochondrial and nuclear loci as well as amplified fragment length polymorphisms.…”