September/October Cover

Submitted by Michi on 11 September 2023.

The September/October cover features the study by Johnsen et al: "Genetic diversity and sperm characteristics are not associated in two bluethroat (Luscinia svecica) populations". The photo of a courting male adult blue throat was taken by photographer Bjørn Aksel Bjerke, close to one of the field sites in the study,  at Øvre Heimdalen in the Jotunheimen mountains in Norway, on May 22 in 2022. Male bluethroats display their beautiful throat and tail colours at close distance to the female in the early parts of the breeding season.



Individual heterozygosity may influence the expression of fitness-related traits, via genome-wide or local genetic effects. Earlier studies have shown negative relationships between heterozygosity and sperm variation, predominantly in captive, highly inbred populations. Little is known about the possible influence of variation in heterozygosity on sperm traits in wild, outbred populations. We studied two populations of the bluethroat, one from the widely distributed northern subspecies Luscinia. s. svecica and the other from the smaller, more patchily distributed subspecies breeding along the French coast of Brittany L. s. namnetum. The two subspecies differed significantly in body size, plumage colour, sperm traits and the degree of genetic diversity. However, there was no evidence that sperm traits (total length and motility) were influenced by the degree of heterozygosity at the individual level. In contrast, we found that male body size was positively related to heterozygosity across both populations, indicating a possible relationship between overall genetic diversity and general vigour or ability to obtain food. We conclude that sperm traits are unrelated to levels of heterozygosity in the studied outbred and weakly genetically depauperate bluethroat populations.