Evolution in Spartina (Gramineae): II. Chromosomes, basic relationships and the problem of S. ×townsendii agg.

Abstract

An intensive study of the cytology and sexual behaviour of the classical Spartina x townsendii agg. and their putative parents in southern England reveals a more complex chromosomal situation than had previously been reported, including aneusomaty in Southampton S. alterniflora populations and in S. x townsendii agg. Chromosome numbers are 2n = 60 and 2n = 62 for S. maritima and S. alterniflora respectively and 2n = 62 and 2n = 120, 122 and 124 (chromosome races) for S. x townsendii F1 and Amphidiploid derivatives respectively. These numbers, though their summation is less precise than hitherto reported, still support the hybridity and amphidiploid origin of S. x townsendii agg. and the process is further confirmed by meiotic pairing data and the discovery of wild backcross hybrids (2n = c.90 and 2n = 76) near the site of origin at Southampton. The 2n = 62 chromosome number of S. alterniflora, out of line with the x = 10 base number of the genus, is explained as a polysomic condition (2n = 60+ 2) and needs further investigation with a study of North American populations. It is suggested that chromosome races found in S. x townsendii Amphidiploid will provide adaptive variation in this highly significant and successful amphidiploid.