Female Choice for Males with Greater Fertilization Success in the Swedish Moor Frog, Rana arvalis

Background: Studies of mate choice in anuran amphibians have shown female preference for a wide range of male traits despite females gaining no direct resources from males (i.e. non-resource based mating system). Nevertheless, theoretical and empirical studies have shown that females may still gain indirect genetic benefits from choosing males of higher genetic quality and thereby increase their reproductive success. Methodology/Principal Findings: We investigated two components of sexual selection in the Moor frog (Rana arvalis), precopulatory female choice between two males of different size (&lsquo;large&rsquo; vs. &lsquo;small&rsquo;), and their fertilization success in sperm competition and in isolation. Females&rsquo; showed no significant preference for male size (13 small and six large male preferences) but associated preferentially with the male that subsequently was the most successful at fertilizing her eggs in isolation. Siring success of males in competitive fertilizations was unrelated to genetic similarity with the female and we detected no effect of sperm viability on fertilization success. There was, however, a strong positive association between a male&rsquo;s innate fertilization ability with a female and his siring success in sperm competition. We also detected a strong negative effect of a male&rsquo;s thumb length on his competitive siring success.Conclusions/Significance: Our results show that females show no preference for male size but are still able to choose males which have greater fertilization success. Genetic similarity and differences in the proportion of viable sperm within a males ejaculate do not appear to affect siring success. These results could be explained through pre- and/or postcopulatory choice for genetic benefits and suggest that females are able to perceive the genetic quality of males, possibly basing their choice on multiple phenotypic male traits.<br /

Ecological genetics of inbreeding, outbreeding and immunocompetence in Ranid frogs

Using artificial fertilization, I crossed frogs from different populations to evaluate fitness consequences for the offspring from an inbreeding-outbreeding perspective, and to evaluate quantitative genetic effects on immunocompetence against a fungal pathogen (Saprolegnia). Crosses between closely situated populations of different sizes generated contrasting results for the effects of outbreeding on offspring traits between populations and life history stages, emphasizing the importance of epistatic effects and the difficulties of relying on generalizations when making conservation decisions (e.g., regarding translocations). Experimental infection of frog eggs from six populations with Saprolegnia fungus showed a significant family effect on the degree of infection of eggs and embryos, in particular at lower fertilization success and with a significant temperature × population interaction effect. A paternal genetic effect on fungus resistance was found using a half-sib split design. Furthermore, relatively more eggs were infected when fertilized by sperm from the same, in contrast with a different population. However, there was no evidence for a stronger effect in isolated island populations. Although the mechanistic underpinnings remain unknown, these results suggest substantial levels of genetic variation in resistance to Saprolegnia in natural populations within and among populations. We also found that pre-hatching exposure to Saprolegnia dramatically reduced the size at metamorphosis in the absence of further exposure to the fungus, possible as a delayed effect of impaired embryonic development. However, in contrast to some other amphibians, induced hatching in response to Saprolegnia could not be confirmed. In conclusion, the results suggest that frog populations are genetically diverse even at small geographic scale with frequently strong and unpredictable consequences of in- and outbreeding for the response to stressors

A genetic component of resistance to fungal infection in frog embryos

The embryo has traditionally been considered to completely rely upon parental strategies to prevent threats to survival posed by predators and pathogens, such as fungi. However, recent evidence suggests that embryos may have hitherto neglected abilities to counter pathogens. Using artificial fertilization, we show that among-family variation in the number of Saprolegnia-infected eggs and embryos in the moor frog, Rana arvalis, cannot be explained by maternal effects. However, analysed as a within-females effect, sire identity had an effect on the degree of infection. Furthermore, relatively more eggs and embryos were infected when eggs were fertilized by sperm from the same, compared with a different, population. These effects were independent of variation in fertilization success. Thus, there is likely to be a significant genetic component in embryonic resistance to fungal infection in frog embryos. Early developmental stages may show more diverse defences against pathogens than has previously been acknowledged

The relationship between the differential siring success of males in sperm competition and (A) their differential fertilization success in controls (<i>F</i>_{2, 20} = 21.2, <i>P</i> = 0.0002) and (B) their differential thumb length (<i>F</i>_{2, 20} = 9.30, <i>P</i> = 0.007).

<p>The relationship between the differential siring success of males in sperm competition and (A) their differential fertilization success in controls (<i>F</i>_{2, 20}  = 21.2, <i>P</i> = 0.0002) and (B) their differential thumb length (<i>F</i>_{2, 20}  = 9.30, <i>P</i> = 0.007).</p

Relationship between thumb length and snout-vent length.

<p>Relationship between thumb length and snout-vent length.</p