Åsa A. Borg

Unusually dynamic sex roles in a fish

Sex roles are typically thought of as being fixed for a given species. In most animals males compete for females, whereas the females are more reluctant to mate. Therefore sexual selection usually acts most strongly on males. This is explained by males having a higher potential reproductive rate than females, leading to more males being sexually active (a male-biased operational sex ratio). However, what determines sex roles and the strength of sexual selection is a controversial and much debated question. In this large-scale field study, we show a striking temporal plasticity in the mating competition of a fish (two-spotted goby, Gobiusculus flavescens). Over the short breeding season fierce male–male competition and intensive courtship behaviour in males were replaced by female–female competition and actively courting females. Hence, sex role reversal occurred rapidly. This is the first time that a shift in sex roles has been shown in a vertebrate. The shift might be explained by a large decline in male abundance, strongly skewing the sex ratio towards females. Notably, the sex role reversal did not occur at an equal operational sex ratio, contrary to established sex role theory.

Seasonal change in female choice for male size in the two-spotted goby

Mate choice may vary according to various factors, such as mate availability and variation in mate quality. We tested seasonal changes in female choice regarding male size in a marine fish, the two-spotted goby, Gobiusculus flavescens. In this species, males occupy nests in which females lay their eggs and males thereafter provide exclusive parental care of the eggs until hatching. Females are predicted to prefer large males if, for example, they are better at competing for and defending nests. In this species, the opportunity for females to be selective should decline over the breeding season as the availability of males decreases. We therefore predicted a reduction in female choosiness with season. We conducted mate choice tests, during the early (May) and later (June) part of the breeding season, in which females were allowed to choose between males differing in body length. Females chose large males early in the season, but became unselective with respect to male size later on. This change in mate choice may be caused by a reduction in overall choosiness as a result of the decline in male availability with season. Alternatively, the change could be caused by a switch in choice cues used by females if factors other than male size become more important for female reproductive success later in the breeding season. Further studies are needed to test between these alternative explanations.

Evidence of inbreeding depression but not inbreeding avoidance in a natural house sparrow population.

Inbreeding is common in small and threatened populations and often has a negative effect on individual fitness and genetic diversity. Thus, inbreeding can be an important factor affecting the persistence of small populations. In this study, we investigated the effects of inbreeding on fitness in a small, wild population of house sparrows (Passer domesticus) on the island of Aldra, Norway. The population was founded in 1998 by four individuals (one female and three males). After the founder event, the adult population rapidly increased to about 30 individuals in 2001. At the same time, the mean inbreeding coefficient among adults increased from 0 to 0.04 by 2001 and thereafter fluctuated between 0.06 and 0.10, indicating a highly inbred population. We found a negative effect of inbreeding on lifetime reproductive success, which seemed to be mainly due to an effect of inbreeding on annual reproductive success. This resulted in selection against inbred females. However, the negative effect of inbreeding was less strong in males, suggesting that selection against inbred individuals is at least partly sex specific. To examine whether individuals avoided breeding with close relatives, we compared observed inbreeding and kinship coefficients in the population with those obtained from simulations of random mating. We found no significant differences between the two, indicating weak or absent inbreeding avoidance. We conclude that there was inbreeding depression in our population. Despite this, birds did not seem to actively avoid mating with close relatives, perhaps as a consequence of constraints on mating possibilities in such a small population.

Variation in MHC genotypes in two populations of house sparrow (Passer domesticus) with different population histories

Small populations are likely to have a low genetic ability for disease resistance due to loss of genetic variation through inbreeding and genetic drift. In vertebrates, the highest genetic diversity of the immune system is located at genes within the major histocompatibility complex (MHC). Interestingly, parasite-mediated selection is thought to potentially maintain variation at MHC loci even in populations that are monomorphic at other loci. Therefore, general loss of genetic variation in the genome may not necessarily be associated with low variation at MHC loci. We evaluated inter- and intrapopulation variation in MHC genotypes between an inbred (Aldra) and a relatively outbred population (Hestmannøy) of house sparrows (Passer domesticus) in a metapopulation at Helgeland, Norway. Genomic (gDNA) and transcribed (cDNA) alleles of functional MHC class I and IIB loci, along with neutral noncoding microsatellite markers, were analyzed to obtain relevant estimates of genetic variation. We found lower allelic richness in microsatellites in the inbred population, but high genetic variation in MHC class I and IIB loci in both populations. This suggests that also the inbred population could be under balancing selection to maintain genetic variation for pathogen resistance.

Low neutral genetic variability in a specialist puffin hunter: the Norwegian Lundehund.

The genetic variability of 125 Norwegian Lundehund and 27 Nova Scotia Duck Tolling Retriever was analysed using a set of 26 microsatellite markers. In Lundehund, the average number of alleles per locus was 1.73, and average observed (H(O)) and expected (H(E)) heterozygosity were 0.07. In Toller, all measures of genetic diversity were much higher than in Lundehund and similar to studies on other dog breeds. The cluster analysis correctly assigned individuals to their respective breed. The low genetic variability in Lundehund was not surprising, given the two strong bottlenecks in the 1940s and the 1960s. The relatedness of Lundehund to other Nordic small spitzes should be investigated in the view of possible outcrossing.

Egg-Laying in Relation to Egg Substrate in Gryon bolivari, an Egg Parasitoid of the Golden Egg Bug (Phyllomorpha laciniata)

Insect female reproductive success is determined by the number of eggs she lays during her life and the survival of her offspring. However, for female parasitoids host availability may limit their reproductive success. Female parasitoids must first locate hosts and then oviposit in them in order to be reproductively successful. In general, this parasitization process has been divided into four behavioral steps: host habitat location, host location, host recognition and host acceptance (Vinson, 1976; Vinson, 1998). Wasps within the Scelionidae family are egg parasitoids of insects as well as other arthropods (e.g. spiders) (see review: Austin et al., 2005). For Scelionids host search and location should be important since the eggs of the host species are relatively small and non-mobile. Gryon bolivari (Giard) (Hymenoptera: Scelionidae) is an egg parasitoid of the golden egg bug, Phyllomorpha laciniata (Villers) (Heteroptera: Coreidae) (Bolivar, 1894; Mineo, 1984; Kaitala, 1996). G. bolivari and P. laciniata coexist in several populations within the Mediterranean area, for instance in Italy (Mineo, 1984) and Spain (Kaitala, 1996; Katvala and Kaitala, 2001).

Genetic variability and structure of the water vole Arvicola amphibius across four metapopulations in northern Norway

Water vole Arvicola amphibius populations have recently experienced severe decline in several European countries as a consequence of both reduction in suitable habitat and the establishment of the alien predator American mink Neovison vison. We used DNA microsatellite markers to describe the genetic structure of 14 island populations of water vole off the coast of northern Norway. We looked at intra- and inter-population levels of genetic variation and examined the effect of distance among pairs of populations on genetic differentiation (isolation by distance). We found a high level of genetic differentiation (measured by FST) among populations overall as well as between all pairs of populations. The genetic differentiation between populations was positively correlated with geographic distance between them. A clustering analysis grouped individuals into 7 distinct clusters and showed the presence of 3 immigrants among them. Our results suggest a small geographic scale for evolutionary and population dynamic processes in our water vole populations.