Maintenance of genetic variation in automictic root-knot nematodes

Differences in amplified fragment length polymorphisms (AFLP) between isolates and between mono-female lines of facultative automictic Meloidogyne hapla race A and obligate apomictic M incognita were determined to test the hypothesis that inverted meiosis occurs. DNA of the parthenogenetic nematode lines were extracted from juveniles, which had been propagated for two generations, allowing males in M. hapla lines to be formed and fertilisation to take place. Based on AFLP analysis, the genetic distance between mono-female lines of M. incognita appeared to be almost nil. By contrast, the genetic distance between isolates and between mono-female lines after seven generations of parthenogenesis of M. hapla was larger. The genetic distance between two mono-female lines of M. hapla, after one generation of parthenogenesis and originating from one line with six generations of parthenogenesis, was larger than the distance between M. incognita lines and the smallest distance between M. hapla lines. The numbers of DNA fragments appeared to be equally variable between lines within a single M. hapla isolate as between various isolates of M. hapla. This maintenance of genetic variation in M. hapla is likely to be caused by the combination of post-reduction in an inverted meiosis, due to chromosomes with diffuse centromeres, and the fusion of the haploid products of the second meiotic division.