Age-related variation in primary sexual characters in a passerine with male age-related fertilization success, the bluethroat Luscinia svecica

摘要

Sperm cells are the most variable animal cells, and a tremendous variation in sperm phenotypes exists among species, from minute amoeboid sperm to giant sperm in some species of fruit flies. Much effort has been devoted to the study of sperm evolution. Sperm competition, when sperm from two or more males compete over fertilization of a set of ova, is thought to be the major force driving the evolution of sperm. There are several ways in which sperm competition can effect the evolution of sperm traits, e.g. through increased sperm production, sperm motility and size.This thesis concerns variation among and function of sperm in passerine birds. At three levels, within males, between males of the same species and between males of different species, co-workers and I have studied variation in sperm morphology and motility of selected species of passerine birds. Sperm traits, such as sperm numbers, sperm size and sperm motility are potentially important for male fertilization success. Hence, sperm traits are expected to be under strong selection due to evolutionary forces such as e.g. sperm competition. Thus, it is surprising how scarce the available information on sperm traits (size, variation, motility) is in passerine birds.In an intraspecific study on bluethroats Luscinia svecica, we found that primary sexual characters, but not sperm size or motility, were age-dependent. These results suggest that increased sperm production in older males can help explain the age-dependent patterns in fertilization success observed in this species. Further, we documented that between-male variation in sperm size is higher than within-male in bluethroats and willow warblers Phylloscopus trochilus, and through resampling procedures we suggested appropriate sample sizes for spermatozoa measured per male and number of males that should be sampled to obtain precise estimates of sperm size and sperm size variation. Moreover, we found that variation in sperm size is closely and negatively related to risk of sperm competition, in a comparative analysis of 22 passerine species. Our interpretation of these results is that increased risk of sperm competition leads to stabilizing selection on sperm size. In another comparative study on 42 passerine species, we documented that sperm swimming speed and sperm length is positively related to risk of sperm competition. However, we found no relationship between sperm swimming speed and sperm size. Thus, it seems that sperm competition leads to both increased sperm swimming speed and increased sperm length, independent of each other. In addition, sperm swimming speed was negatively related to clutch size, a proxy for female sperm storage. Finally, we found in an intraspecific study on tree swallows Tachycineta bicolor, that both sperm quantity and quality were significantly related to fertilization success. This result has great implications as the first study linking sperm traits and fertilization success in a free living passerine.