The southern hemisphere has been divided into six Areas for baleen whale management purposes. It was assumed that of the different baleen whale species one population lives in each Area. However, evidence exists which suggests that different Fin Whale populations intermingle on the feeding grounds of anArea. This obviously would have implications with respect to Fin Whale management.
Interaction of the Columbia root-knot nematode, Meloidogyne chitwoodi, with
Brassica spp. was studied in pot and in Petri dish experiments for 11
nematode isolates and three F1 hybrids of cauliflower, broccoli and
rapeseed. In the pot experiment, the host status of the different Brassica
hybrids varied greatly (average of 4-45 egg-masses). The isolate-by-cultivar
interaction effect was significant but depended only on three exceptions to
the generally equal patterns for all isolates. In the Petri dish experiment,
this large variation in aggressiveness was confirmed. Cauliflower, as a host
for M. chitwoodi, is less favourable than rapeseed, and rapeseed is less
favourable than broccoli. Several mechanisms are involved in the
host-parasite interaction: limitation in penetration, in juvenile
development, and in female-male ratio, each one acting with a high level of
specificity to the isolates tested. No hypersensitive reaction was observed.
In view of this quantitative resistance reaction, the species M. chitwoodi
expresses an extremely large variability in terms of aggressiveness.
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.
Variability for pathogenicity on seven Solanum bulbocastanum clones was studied in a collection of isolates of Meloidogyne chitwoodi and M. fallax from a wide range of geographical origins. Four different pathotypes could be distinguished within M. chitwoodi, while no specialisation was found in M. fallax. Meloidogyne chitwoodi isolates from the USA showed the largest variation; those from Europe belonged to one pathotype. In several M. chitwoodi isolates, the occurrence of pathotype mixtures was verified and for the first time reported in automictic Meloidogyne spp. The infrequently formed egg masses in incompatible isolate-plant genotype combinations were not able to reproduce further on various S. bulbocastanum genotypes, suggesting some stability aspects of the resistance. The occurrence of pathotypes and mixtures of pathotypes requires special attention in breeding programmes as well as in disease management.