Publication Date

1989

Description

Since_ the late 1970's, many medic based pasture leys in Australia have been consistently and severely damaged by both blue green aphid (BGA) (Acyrthosiphon kondoi) and to a lesser extent spotted alfafa aphid (SAA) (Theroaphis trifolii fm maculata). Where infested stands are not sprayed, seed set has been severely affected, and this seed set is vital if the system is to ?ea successfull self-regenerating one (Carter, 1981). Aphid resistant cultivars are likely to provide a long term answer to this problem, and since the early 1980's several new resistant barrel medic (M. truncatula) cultivars have been released. How­ e:'er, other species are widely grown and better adapted to sig­mficant areas of the Australian medic zone. Strand medic (M. littoralis) and disc_ medic (M. tornata) are two such species. Exhaustive screenmg of the extensive collections of these species held within the South Australian Department of Agriculture's National Annual Medicago Collection has failed to yield accessions with good levels of resistance to either BGA or SAA. However, as M. truncatula is quite closely related to both species (Small and Lefkovitch, 1986) attempts were made to produce interspecific hybri_ds between them and M. trunca­tula to enable resistance transfer. Successful hybrids between an SAA and BGA resistant M. truncatula line (SA10419) and M. littoralis (cv Harbinger) were produced. These showed c nsiderable genome incompatibility m the form of partial _ _stenhty _ and a high frequency of albino and semi-albino plants, but plants resistant to both SAA and BGA were recovered. These in turn were backcrossed to Har­binger and reselected for aphid resistance through several cycles. This has resulted in the production of several lines very smular to Harbinger but with aphid resistance derived from SA10419 (Lake et al., 1987a). In contrast, attempts to hybridize aphid resistant M. trunca­tula lines with various M. tornata accessions were largely unsuccessful. Of several hundred crosses in various combina­tions, only one hybrid was produced, and this proved to be mviable. However, successful hybrids between M. littoralis and M. tornata have been made in the past (Simon, 1965) and hence M. littoralis may be a useful bridging species to introduce resistance to both SAA and BGA into M. tornata from M. truncatula. This paper describes the results of such attempts.

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Transfer of Genes for Aphid Resistance from Medicago truncatula (Barrel medic) via M. Littoralis (Strand Medic) to M. Tornata (Disc Medic)

Since_ the late 1970's, many medic based pasture leys in Australia have been consistently and severely damaged by both blue green aphid (BGA) (Acyrthosiphon kondoi) and to a lesser extent spotted alfafa aphid (SAA) (Theroaphis trifolii fm maculata). Where infested stands are not sprayed, seed set has been severely affected, and this seed set is vital if the system is to ?ea successfull self-regenerating one (Carter, 1981). Aphid resistant cultivars are likely to provide a long term answer to this problem, and since the early 1980's several new resistant barrel medic (M. truncatula) cultivars have been released. How­ e:'er, other species are widely grown and better adapted to sig­mficant areas of the Australian medic zone. Strand medic (M. littoralis) and disc_ medic (M. tornata) are two such species. Exhaustive screenmg of the extensive collections of these species held within the South Australian Department of Agriculture's National Annual Medicago Collection has failed to yield accessions with good levels of resistance to either BGA or SAA. However, as M. truncatula is quite closely related to both species (Small and Lefkovitch, 1986) attempts were made to produce interspecific hybri_ds between them and M. trunca­tula to enable resistance transfer. Successful hybrids between an SAA and BGA resistant M. truncatula line (SA10419) and M. littoralis (cv Harbinger) were produced. These showed c nsiderable genome incompatibility m the form of partial _ _stenhty _ and a high frequency of albino and semi-albino plants, but plants resistant to both SAA and BGA were recovered. These in turn were backcrossed to Har­binger and reselected for aphid resistance through several cycles. This has resulted in the production of several lines very smular to Harbinger but with aphid resistance derived from SA10419 (Lake et al., 1987a). In contrast, attempts to hybridize aphid resistant M. trunca­tula lines with various M. tornata accessions were largely unsuccessful. Of several hundred crosses in various combina­tions, only one hybrid was produced, and this proved to be mviable. However, successful hybrids between M. littoralis and M. tornata have been made in the past (Simon, 1965) and hence M. littoralis may be a useful bridging species to introduce resistance to both SAA and BGA into M. tornata from M. truncatula. This paper describes the results of such attempts.