Chromosome Number and Behavior in Arctic Mosses

Abstract

To obtain independent data bearing on the hypothesis that the frequency of polyploidy increases with latitude, the chromosome number and meiotic behavior of mosses in arctic Alaska were investigated. Chromosome numbers were determined at meiosis in 55 spp., representing 35 genera and 18 families, of which 44 spp., 10 genera, and 1 family had not received previous cytological attention. Precise temp. data taken 2 mm. and 2 cm. above the surface of a frost mound provide material for a more exact understanding of the microenvironment in which arctic mosses produce vegetative and reproductive structures. The lowest chromosome number found was n = 6; the highest, n = 50. Polyploidy occurs widely in arctic mosses but is no more frequent statistically than in comparable populations of mosses in California and Finland. No polyploidy arctic races were found within species whose chromosome numbers have been reported in populations from more temperate regions. Several typically arctic species are highly polyploid and have chromosome numbers higher than those previously reported in their family; however, other typically arctic species showed no such increase. Aneuploidy seems to be lacking in the arctic species investigated. Minute bivalent chromosomes, whose component parts dissociate prematurely and may divide precociously, occurred in 22 of 55 spp., which are presumably tetraploids or of higher ploidy. In view of the consistent behavior of these tiny chromosomes and their regular appearance in mosses, the terms "accessory chromosomes" or "isochromosomes" cannot properly be applied to them. No higher degree of abnormal meiosis was observed in the arctic mosses than in more temperate regions, in spite of the more severe temperature regime. The chromosome numbers of the various arctic families are compared, and the effect of proportional representation of different families on statistical comparisons is noted. The different habitats of acrocarpous and pleurocarpous mosses result in a higher percentage of the former at higher latitudes, with concomitant effects on chromosome numbers and frequency of polyploidy. It is concluded from this random sampling of chromosome numbers of arctic mosses that the frequency of polyploidy does not increase automatically with an increase in latitude, as has been postulated for higher plants.