Polyploidy in the evolution of the glacial relict Pontoporeia spp. (Amphipoda, Crustacea)

S alemaa , H. 1984. Polyploidy in the evolution of the glacial relict Pontoporeia spp. (Amphipoda, Crustacea). — Hereditas 100: 53–60. Lund, Sweden. ISSN 0018–0661. Received April 19, 1983 The chromosome numbers and karyological features were investigated in two benthic amphipods, Pontoporeia femorata Kröyer, and Pontoporeia affinis Lindström, from the northern Baltic Sea. A considerable difference was observed between the chromosome numbers of the species. The diploid karyotype of P. femorata consists of six acrocentric and twenty‐two metacentric and submetacentric elements, the fundamental number of chromosome arms being 50 (n = 14). P. affinis has evolved from P. femorata in the Ice Age through polyploidy and further chromosome fusions, the number of acrocentric elements being eight and that of metacentric and submetacentric chromosomes forty‐four. There are 96 arms in the karyotype of P. affinis (n=26). No chromosomal mechanism for sex‐determination was observed. Because polyploidy is a rare event in bisexual animals, being demonstrated for the first time in Amphipoda, an attempt has been made to elucidate polyploidy as an evolutionary strategy. Compared to its ancestor, P. femorata, P. affinis is characterized by ecological flexibility, high but fluctuating productivity and increased reproductive effort. These features, together with the zoogeographical history of glacial relicts in northern Eurasia, support the theory that polyploid animals are often generalistic pioneers, easily invading new habitats in peripheral or changing environmental conditions.