Development and characteristics of triple species hybrids used to transfer reniform nematode resistance from Gossypium longicalyx to Gossypium hirsutum

Two triple-species hybrids were developed by a protocol designed to achieve a tetraploid plant (52 chromosomes) containing one complete set of 13 F genome chromosomes from Gossypium longicalyx, with the goal of backcrossing the hybrids with G. hirsutum to introgress genes from the F genome into Upland cotton. The genes of greatest interest were those conferring immunity to the reniform nematode, an important trait found in no other Gossypium species. The strategy was to substitute an F genome set of chromosomes for an A genome set in a triple- species hybrid of G. hirsutum x G. longicalyx x G. amourianum (HLA), and for a D genome set in a triple-species hybrid of G. hirsutum x G. herbaceum x G. longicalyx (HHL). The HLA hybrid was formed by crossing G. armourianum pollen onto a G. hirustum 'TM-1' (Deltapine 14) x G. longicalyx hexaploid created by Dr. Meta Brown. The HHL hybrid was formed by crossing G. longicalyx pollen onto a G. hirsutum 'Tamcot CAMD-E' x G. herbaceum hexaploid. Both hexaploids were obtained by colchicine treatment of vegetative buds of their triploid progenitors. The hybrids retained R. reniformis immunity. Nematodes infected roots but neither hybrid supported normal development or egg mass production by parasitic females on roots, and nematode populations in soil ultimately dropped to undetectable levels when hybrids were grown in pots inoculated with nematodes. The hybrids exhibit various parental traits related to growth habit and flower morphology. Both hybrids are male sterile but flowers can be pollinated by G. hirsutum to obtain backcross progeny, with six times as many viable embryos obtained from HLA as HHL. More than 1,100 progeny have been obtained from the hybrids, with ca. 900 characterized in relation to fertility and (or) nematode resistance. Because a small number of nematodes from the primary inoculum usually survive until the termination of nematode resistance assays, a value of 1% of the nematodes observed either in soil or on roots of the susceptible 'Deltapine 16' control was operationally defined as immunity. Among 66 BC1 progeny of HLA with various male parents, 9 were scored as immune and 8 as either immune or highly resistant. Among BC1 populations obtained by crosses of 'Acala NemX' pollen onto HLA (603 progeny) and HHL (21 progeny), 106 and 4 plants, respectively, were male-fertile with 12 and 1 plant, respectively, scored as immune to R. reniformis. Immunity was pollen-transferred to one-fourth (26%) of progeny within second and third G. hirsutum backcross generations from 'Delta and Pineland 458' bolls, suggesting inheritance by a single dominant gene.