Submitted to: Euphytica
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/3/2003
Publication Date: 1/20/2004
Citation: Boiteux, L.S., Hyman, J.R., Bach, I.C., Fonseca, M.N., Matthews, W.C., Roberts, P.A., Simon, P.W. 2004. Employment of flanking codominant STS markers to estimate allelic substitution effects of a nematode resistance locus in carrot. Euphytica. 136:37-44. Interpretive Summary: Several years ago we discovered a gene which protects carrots against soil borne nematode pests. This gene can be present in 0, 1, or 2 copies in each plant but it was not known how much more resistance to nematodes could be realized with 2 copies as compared to 1 copy. In this study we demonstrated that while one copy of the gene significantly reduces nematode attack, two copies provides much better protection against nematodes. This information is of interest to carrot geneticists, breeders, and growers who develop and utilize genetic resistance in nematodes.
Technical Abstract: In carrot, two codominant sequence-tagged site (STS) markers, flanking in tight linkage the Meloidogyne javanica resistance (Mj-1) locus, were employed to investigate the association between expression of resistance and locus dosage. Phenotypic expression of homozygous resistant (R); heterozygous; and homozygous susceptible (S) individuals in an F2 population of 396 F2 plants from 'Brasília-1252' (R) x 'B6274' (S) was estimated for three resistance criteria: total egg production per plant (TEP), egg production per gram of fibrous root (EPG) and root gall index (RGI). The homozygous resistant class had average values of 403.9 for TEP; 147.5 for EPG and 0.8 for RGI. The heterozygous class had 1,673; 477.3; and 0.16 whereas the homozygous susceptible class had 68,604; 11,877; and 2.54, respectively. The dominance ratio (d/a) indicated that genomic region(s) derived from the resistant parent encompass genetic factor(s) with almost complete dominance for RGI (d/a = 0.93-0.94) and incomplete dominance for transformed (TEP)0.25 and (EPG)0.25 (d/a = 0.63-0.65). Broad sense heritabilities were high varying from 72.9% for (EPG)0.25 to 86.0% for RGI. Narrow sense heritability values ranged from 55.9% for RGI to 64.3% for (TEP)0.25 . Highly significant orthogonal contrasts were observed between homozygous resistant vs. heterozygous for (TEP)0.25 and (EPG)0.25 . Marker-assisted selection could greatly facilitate the incorporation of the Mj-1 allele in both male-fertile and male-sterile counterpart lines in order to obtain F1 hybrids with the most effective levels of resistance.