Chromosome Number of the Potato Root Eelworm, Heterodera rostochiensis Wollenweber

Abstract

Riley and Chapman1 have recently reported that the diploid chromosome number of the female potato root eelworm is 18. During the course of a comparative study of the cytology of the genus Heterodera, I have found from a study of eggs undergoing meiosis that although this number appears to be usual in the potato root eelworm, eggs having 20 and 22 as the diploid number are not uncommon, while occasional eggs with 19, 23 and 24 may also occur. Material was fixed in Carnoy's fluid and stained with 1 per cent orcein in 45 per cent propionic acid using a squash technique. The distribution of chromosome number shown in Fig. 1 is based on 40 eggs from 32 females. In most cases where it has been possible to determine the chromosome number for more than one egg per female the determinations have agreed, but it is not certain whether this is always the case. The chromosomes are approximately 1µ long, so that counts may not be accurate in all cases; nevertheless it is clear that the number of chromosomes varies.