Segregation for Double Spine Trait in Hybrids of Microcitrus inodora

Abstract

Several Microcitrus species are valuable as parents in breeding citrus rootstocks and scions because of useful characteristics, such as monoembryony, very short juvenility, short fruit maturation time, and resistance to Phytophthora and nematodes. The large-leaf Australian wild lime, Microcitrus inodora (F. M. Bail.) Swing., also possesses a trait that is generally undesireable in new hybrids, double axillary spines. Most F1 hybrids between M. inodora and single-spine Microcitrus and Citrus species were observed to possess the double spine trait. In crosses of M. australasica (F. Muell.) Swing. × M. inodora, M. papuana H. F. Winters × M. inodora, and M. warburgiana (F. M. Bail.) Tan. × M. inodora, double spines were present in 80%, 81%, and 87% of the progeny, respectively. In hybrids of Citrus species with M. inodora, percentage of double spines varied from 43% in progeny from M. inodora × Citrus aurantium L., to 100% in progeny from M. inodora × Citrus ichangensis Swing. When double-spined hybrids of M. inodora were used as parents in crosses with other single-spined Citrus species or hybrids, progenies varied from 0 to 86% with double spines. The patterns of inheritance suggest that the double spine trait is controlled by an allele of a nuclear gene that is dominant and homozygous in M. inodora, but that one or more other independently segregating alleles are necessary for expression of the double spine trait in advanced generation hybrids.