Genetic variation of Zoarces viviparus: six populations revisited after about 35 years

摘要

Eelpout, Zoarces viviparus, has been used as study object for nearly 100 years by several Danish scientists. The advantage of eelpout is that it is viviparous which means that the genetic contribution from the parents to the offspring may be recognised and identified when the genotype of the female is known. Already in 1917 Schmidt published a work on the number of rays in the right pectoral fin, vertebrate number, number of hard rays in the dorsal fin and number of pigmentation spots along the dorsal fin. The first conclusion based on vertebrate number, hard rays and pigmentation spots was that environmental factors per se was not sufficient to explain the similarity between the female and the offspring, so a genetic component had to be involved in the determination of the four traits. Secondly, four major groups of eelpouts were recognised, i.e. one group living in the western part of the North Sea, one in the eastern part of the North Sea, one in the western part of the Baltic Sea encompassing the internal Danish waters, and one in the eastern part of the Baltic Sea. Thirdly, a gradient in the traits was found in four fjords. Mainly one of the fjords, Mariager Fjord, has been studied several times. Samples were analysed by using the analytic method described by Schmidt (1917) and by isozymes (Christiansen et al. 1981, 1988; Olsen et al. 2002). Christiansen et al. (1981, 1988) found that the morphological traits were nearly unchanged for at least 60 years and some of the isozymes showed the same gradient as the morphological traits which indicated that the fjord harboured at least two populations of eelpouts, one in the inner part and one in the outer part of the fjord. However, after a tremendous oxygen deficiency in 1998 where nearly all life in the fjord was destroyed, both the morphological traits and the isozymes were revealing gradients indicating that the fjord was re-colonised from the population located outside of the fjord (Olsen et al. 2002).During the 1970s a number of eelpouts were analysed by means of isozymes. The major conclusion was that the eelpouts in the Danish waters were divided into more groups than recognised by Schmidt (1917) (Christiansen et al. 1976; Simonsen and Christiansen 1985). By the end of last century and now, several studies on deformities among the larvae were published (Vetemaa et al. 1997; Strand et al. 2004; Gercken et al. 2006). Previous studies on eelpouts in the breeding season in the period 1969–1974 (Christiansen et al. 1977) collected close to Aarhus in the southern part of Kattegat revealed only a few dead individuals among the investigated litters and no deformities were observed (unpubl.). Furthermore, sampling in the breeding season 1975–1977 at a locality 30 km south of Aarhus did not reveal any deformities (unpubl.).The aim of the present study was to see if the genetic distribution in samples collected to day was deviating from the distribution found in the 1970s and if the lately recognised impaired larval development was reflected in the genotypic distribution among the eelpouts.