The number of tepary bean AFLP bands introgressed into the congruity backcross lines was compared with the number of tepary bean AFLP bands found in a group of cultivars and lines developed from recurrent backcrossing. The relative advantage of recurrent versus congruity backcrossing is discussed in terms of the relative investment in effort for these two methods, the level of introgression obtained, and the possibility of transferring complex traits such as drought tolerance from tepary bean to common bean.
A third set of genotypes included interspecific lines and cultivars derived from standard recurrent backcrossing and selection.
The first group contained all cultivars or advanced lines without tepary bean parentage (ICA Pijao and BAT41) and all the interspecific hybrids from a single or multiple recurrent backcrossing. In this group, the B[C.sub.1][F.sub.5] simple congruity backcross lines were all related to each other and to ICA Pijao, the recurrent common bean parent, as well as to the recurrent backcross interspecific lines in the VAX or XAN series, the cultivars Tara and Jules and the common bean lines such as MAR1 and BAT41.
To reach this goal, hexaploids can be used either directly through recurrent backcrossing
to the tetraploid parent (Brown and Menzel, 1950) or indirectly through the development of trispecific allotetraploid hybrids with A- or D-genome diploid bridging species (Deodikar, 1949).
The simultaneous introgression of two genes from different sources into the genetic background of one recipient genotype by recurrent backcrossing
is a common task, for example, in the development of inbred lines for hybrid varieties or line cultivars in autogamous species.
Marker-assisted foreground selection was proposed by Tanksley (1983) and investigated in the context of introgression of resistance genes by Melchinger (1990), who presented an a priori approach for calculating the minimum number of individuals and family size required in recurrent backcrossing
. Marker-assisted background selection was proposed by Young and Tanksley (1989) and investigated by various authors (Hospital et al., 1992; Openshaw et al., 1994; Visscher et al., 1996; Frisch et al., 1999a,b).
The objectives of the study reported in this paper were to improve the BB resistance of Minghui 63 by introgressing Xa21, a gene that is highly resistant to a broad spectrum of the pathogen races (Khush et al., 1990), by means of MAS in the process of recurrent backcrossing; and to evaluate the effects of such improvement on the agronomic performance of Minghui 63 and the hybrid under both BB stressed and nonstressed conditions.
The Xa21 gene was introgressed into Minghui 63 following a recurrent backcrossing procedure, combined with tandem selection using molecular markers.
RECURRENT BACKCROSSING is a breeding method commonly employed to transfer alleles at one or more loci from a donor to a recurrent parent (Allard, 1960).
Molecular markers are used in recurrent backcrossing for two purposes: (i) as a diagnostic tool for tracing the presence of a target allele, for which direct selection is difficult or impossible (e.g., recessive alleles expressed at a late stage in plant development or quantitative trait loci) and/or (ii) for identifying individuals with a low proportion of the undesirable genome from the donor parent.
Complete expression of tolerance from Atlas 66 was not achieved via recurrent backcrossing