Employment of flanking codominant STS markers to estimate allelic substitution effects of a nematode resistance locus in carrot

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 F-2 population of 396 F-2 plants from 'Brasilia-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 F-1 hybrids with the most effective levels of resistance.