Inheritance and inter-relationship of fitness characters in progeny of an aggressive × non-aggressive cross of Ophiostoma ulmi

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The inheritance patterns and inter-relationships of nine potential fitness characters were studied in 30 F 1 progeny of a cross between the evolutionarily divergent aggressive (NAN) and non-aggressive subgroups of Ophiostoina ulmi . Continuous inheritance was demonstrated for elm bark colonizing ability and for in vitro ceratoulmin wilt toxin production, and confirmed for linear growth rate at 20 °C and for vascular wilt ability on elm. Inheritance of mycelial density on elm sapwood discs was skewed towards the NAN parental type whereas that for synnemetal and protoperithecial production was skewed towards the non-aggressive type. Growth-temperature response and fertility of the progeny as sexual donors (♂) on a NAN recipient (♀) both followed a bimodal inheritance pattern coinciding with the parental types, the NAN-like type being predominant in each case. As recipients (♀) the majority of the progeny were sexually sterile, a proportion showed a NAN-like response in strongly rejecting the non-aggressive as donor, and one showed a unique response in being more fertile with the non-aggressive donor than with the NAN. Only growth-temperature response and vascular wilt ability were correlated in the F 1 s, where progeny with a non-aggressive type temperature response were shown to be less pathogenic than those with a NAN-type response. The results further explain the lack of aggressive x non-aggressive hybrids in nature. Many of the progeny exhibited poor performance in potential fitness characters such as vascular wilt ability and bark colonizing ability, or were sexually sterile. Progeny showing a good performance in some characters were detected, but since the characters were inherited mainly at random, the chance of a hybrid genotype being sufficiently fitted overall to compete with the aggressive subgroup must be low. Nevertheless, there may be some potential in experimentally pursuing the breeding of a hybrid form of O. ulmi which could compete with the aggressive subgroup in nature.