Salemaa 1984

Salemaa H. 1984. Polyploidy in the evolution of the glacial relict Pontoporeia spp. (Amphipoda, Crustacea). Hereditas 100: 53-60.

This author examined the karyotypes of two closely related species of amphipods found in the Baltic sea and other locations in northern Europe. One species is considered to have descended from the other, speciating about 100 000 years ago during a glacial maximum. The more recent species is tetraploid, but shows normal meiosis and evidence of crossing-over. The two species occur in sympatry, despite apparent ecological congruence.

The tetraploid species shows several differences from the diploid that may or may not relate to its increased chromosome number. They use greater habitat diversity, including a microhabitat difference that exposes them to increased predation from fish. The tetraploids show higher but more variable productivity in some locations where both species are found. Population density of the tetraploids varies greatly; some locations included up to 10 000 individuals per square meter of muddy benthos. The life history of the tetraploid includes more, smaller eggs (contrary to my expectations) and may be better at seasonal synchronization for breeding, based on improved visual sensitivity that allows deeper-dwelling populations to react to season variation in light levels.

All of these differences may have evolved after the speciation event that split this lineage. In particular, greater DNA content per cell should, all else being equal, lead to larger cells including embryos. The opposite difference suggests adaptations of life history to either unmeasured different environmental factors or to the larger cell nuclei of the tetraploids.

This is the first report of a dioecious tetraploid amphipod; no sex chromosomes were detected, removing the obstacle to polyploidy inherent in chromosomal sex determination.

While the karyotype is strongly suggestive of a polyploid recent ancestor followed by some centric fusions (the chromosome number is not exactly double), the illustrations of karyotype do not appear to show longer total chromatin. Genome size, as opposed to karyotype, is not reported here for either species; a doubling of cellular DNA contents would be more convincing to me for an argument of recent polyploidy.