Ancestral sperm ecotypes reveal multiple invasions of a non-native fish in northern Europe

For externally fertilising organisms in the aquatic environment, the
abiotic fertilisation medium can be a strong selecting force. Among bony
fishes, sperm are adapted to function in a narrow salinity range. A
notable exception is the family Gobiidae, where several species reproduce
across a wide salinity range. The family also contains several wide-spread
invasive species. To better understand how these fishes tolerate such
varying conditions, we measured sperm performance in relation to salinity
from a freshwater and a brackish population within their ancestral
Ponto-Caspian region of the round goby, Neogobius melanostomus. These two
ancestral populations were then compared to nine additional invaded sites
across northern Europe, both in terms of their sperm traits and by using
genomic SNP markers. Our results show clear patterns of ancestral
adaptations to freshwater and brackish salinities in their sperm
performance. Population genomic analyses show that the ancestral ecotypes
have generally established themselves in environments that fit their sperm
adaptations. Sites close to ports with intense shipping show that both
outbreeding and admixture can affect the sperm performance of a population
in a given salinity. Rapid adaptation to local conditions is also
supported at some sites. Historical and contemporary evolution in the
traits of the round goby sperm cells is tightly linked to the population
and seascape genomics as well as biogeographic processes in these invasive
fishes. Since the risk of a population establishing in an area is related
to the genotype by environment match, port connectivity and the ancestry
of the round goby population can likely be useful for predicting the
species spread.