The genetic relationships of pathotypes of <i>Heterodera rostochiensis</i> Woll. which reproduce on hybrid potatoes with genes for resistance

Abstract

SUMMARY The hypothesis is advanced that the major genes for resistance to Heterodera rostochiensis in potatoes bred from Solanum tuberosum ssp. andigena (Ab ), from S. multidissectum (aB ) and from both ( AB ) are matched by recessive genes in the nematodes able to overcome resistance, The hypothesis postulates the existence of a pathotype that multiplies only on ab plants (susceptible commercial potatoes) and of one that multiplies on plants with both genes ( AB ). Evidence for the first is of a negative kind but there is positive evidence for the second. When the results of pot tests were combined with tests previously made by Jones &amp; Pawelska (1963), and the forty populations were divided into four groups, the proportions of pathotypes in two groups fitted expectation. Proportions also fitted expectation for the other two groups and for the means of all forty populations when an adjustment was made to allow for the excessive number of cysts formed on plants ex andigena (Ab ). The three groups of populations with appreciable numbers of the pathotype able to reproduce on AB plants discriminated between AB plants with and without an additional factor for resistance derived from S. multidissectum . This indicates the existence of yet another pathotype. AB plants with the additional resistance were about equally as resistant as S. vernei to all forty populations. Assuming that males of all genetic constitutions ( AA, Aa, aa ) can mature in the roots of resistance plants but only recessive females ( aa ) can mature, the proportion of aa progeny should rise to 50 % at the end of the second generation. Pot experiments fit expectation but, in field plots, persistence of the old population delays increase in the proportions of recessive individuals. The growing of plants with one dominant gene for resistance increases the proportions of the pathotype that can reproduce upon them and also that which can reproduce on plants with both genes. The hypothesis parallels the gene‐for‐gene theory advanced to explain the inheritance of the ability of certain rust fungi to overcome the resistance of their hosts but, when applied to Heterodera rostochiensis , has to take into account the faculty of larvae to become either male of female according to circumstances.