29.4. Phenotypic and genetic variations in reproduction-associated traits in Arctic charr, Salvelinus alpinus

Public examination of a doctoral dissertation in the field of biology

Doctoral candidate: M.Sc. Matti Janhunen

Date and venue: 29.04.2011, at 12 noon, N100, Natura, Joensuu campus


Reproductive traits are strong determinants of fitness and are therefore likely to be subject to particularly intense selection. The interplay between natural and sexual selection shapes the evolution of sex-specific characteristics that ultimately determine the breeding success of individuals, i.e. the ability to pass on their genes to the next generation. In order to understand these patterns better, in this thesis, I have addressed some fundamental questions regarding the genetic and phenotypic aspects of traits that may be of importance for reproductive biology. As a model species, I used the Arctic charr, Salvelinus alpinus, an externally fertilizing fish with a non-resource-based mating system.


A comparison of external body shapes between three ecologically distinctive charr populations revealed significant genetically determined differences both outside and during the reproductive season. Sexually matured fish, however, exhibited uniform patterns of sexual dimorphism among the populations. The males possessed a more robust (deeper) body profile, larger head dimensions and longer pectoral fins than females. Such relatively enlarged features of males presumably reflect their more intense intra-sexual breeding competition compared to females.


Controlled breeding experiments indicated that embryo survival and larval post-hatching size are largely attributable to maternal effects. Some of the variations were also explained by independent male effects and the genetic interactions between parents, suggesting that selection during the early stages of development can simultaneously target both intrinsically high-quality and compatible genotypes. The present results also show that increased investment by both sexes at the gamete level can contribute to developmental success. In males, sperm swimming speed was positively associated with progeny viability (consistent with the ‘good sperm’ hypothesis), whereas in females the positive effects were mediated through increased and less variable egg size. Polyandrous mating as a result of sperm competition improved the average survival rate and increased yolk size among offspring above the average values obtained from single matings. Thus, besides showing pre-mating selectivity, females may incur indirect fitness benefits by exposing the eggs to multiple males in a given spawning event. Instead, I found no evidence of consistent viability differences between half-sibling progenies when the charr females were paired with the males they preferred compared to those they did not prefer in a preceding, dichotomous ‘free choice’ test.


The degree of carotenoid-based breeding colouration proved to be a potential indicator of reproductive quality (fertility) within both sexes. In accordance with the phenotype-linked fertility hypothesis, I observed a positive link between male redness and sperm velocity. Thus, brightly coloured males may advertise their efficient fertilization capability to females. Nevertheless, differences in the ornamental expression of male charr did not translate into differences in offspring early fitness traits. In contrast, the intensity of colouration in females was negatively related to their fecundity and brood viability, which refers to a significant trade-off between offspring production and ornamentation. These findings do not support the direct (mutual) selection hypothesis as an explanation for the existence of carotenoid-based breeding displays in both sexes. Instead, the ornamental colouration in female charr might have evolved as a genetic correlative trait resulting from sexual selection on male colouration. However, varying female preferences, selection on genetically compatible males and cryptic female choice may also weaken the directional selection on male colouration and thus maintain its genetic variation within populations of Arctic charr.


The doctoral dissertation of Matti Janhunen entitled Phenotypic and genetic variations in reproduction-associated traits in Arctic charr, Salvelinus alpinus will be examined at the Faculty of Science and Forestry. The opponent in the public examination is Professor Jarle Nordeidend the custos is Professor Raine Kortet of the University of Eastern Finland., University of Nordland, Bodø a

Photo available for download at http://www.uef.fi/vaitoskuvat

Contact: Matti Janhunen, tel. +358 40 567 9868, matti.janhunen@rktl.fi




Publishing year: 2011

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