The Correct Chromosome Number for Crocus medius Balb.

Abstract

In the list of chromosome counts for the genus Crocus (Brighton, C. A., Mathew, B. F. & Marchant, C. J. in Kew Bull. 28(3): 451-464 (1973)) counts of 2n = 28 and 2n = 30 were reported for two collections of Crocus medius from Monte Alto and the Ligurian Alps in Italy. A more detailed study of the karyotypes has shown that B-chromosomes occur in this species and that one pair of chromosomes has very distinct secondary constrictions giving the impression of two separate chromosomes lying close together when in fact there is only one. The correct numbers for the plants examined are 2n = 24 + 2B and 2n = 24 + 5B respectively. Both karyotypes are illustrated in Figure IA & B and consist of three pairs of median chromosomes, one pair of chromosomes with two secondary constrictions, one pair of sub-median chromosomes and seven pairs with subterminal constrictions. From 2-5 B-chromosomes were also present in the two collections examined. Pair IV of both karyotypes has three distinct constrictions (arrowed). It is uncertain which of these is the centromere, but meiotic studies might give some indication. Chromosomes of this kind are very unusual. If one supposes the centromere to be the central constriction, then there is a possibility that these could be iso-chromosomes similar to those found in Nicandra by Darlington & JanakiAmmal in Ann. Bot. II, 9(35): 267-28 (1945). Here again, meiosis would need to be observed in detail to see whether these chromosomes pair normally, or whether each chromosome consists of identical arms which are capable of pairing together.