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Despite the significant progresses achieved in
plant breeding programs, there are still many challenges in order to develop
new varieties with high productivity. Genetics was the greatest
technique in plant breeding programs by means of conventional breeding method,
mutation breeding, molecular marker assisted selection, genetic engineering or
even genome editing and some of these methods have been successfully used in
mungbean breeding (Chaitieng et al., 2002a; Ngampongsai et al., 2009). However, genetic
engineering and genome editing are not available in Thailand. In addition, pyramiding
genes using conventional methods is still difficult due to the dominance and
epistasis effects of genes, time-consuming, laborious as well as dependence of
environmental influences. Molecular markers can be used to pyramid desirable
and multiple genes into the recommended varieties.                                                      Genetic
diversity of the genetic resources is an important first step in any plant
breeding program. The genetic differences of plant genetic resources (PGRs) have been analyzed using
morphological traits or even physiological traits. However, to minimize the impact
of environmental factors, biochemical techniques such as isozyme and protein
markers were later utilized. Since 1990, various molecular techniques such as
restriction fragment length polymorphism (RFLP), random amplified polymorphic
DNA (RAPD), amplified fragment length polymorphism (AFLP), microsatellites or
simple sequence repeat (SSR), inter-SSR (ISSR), which called structural marker,
have been used to assess genetic variation (Islam et al. 2015; Sony et al. 2012; Bhat et al., 2005; Wang
et al., 2017; Nath et al., 2017). Recently, sequence databases of DNA,
cDNA and expressed sequence tag (EST)
or gene-based SSR (genic SSR) have been generated by next generation sequencing
(NGS) for several plants including mungbean
and have been available for screening SSRs to develop EST-SSR markers. Because
they are physically linked to expressed genes, but the traditional genomic SSRs
derived from throughout the genome and identification of them are more costly
and time-consuming than EST-SSRs, they are useful for analyzing the functional
diversity in genetic resources, particularly high yielding and resistant
varieties. (Varshney et
al., 2005). Thus, the genetic
resources with appropriate traits in induced crosses can be used as parents through
this estimation. In addition, the identification of resistance genes against
CLS and PM in genetic resources is helpful for developing resistant varieties. Genetic
studies of inheritance for CLS resistance, using different resistant sources,
indicated that the resistance is controlled by either a single dominant gene (Thakur et al., 1980; Chankaew et
al., 2009; Singh et
al., 2017), or a single recessive gene (Mishra et al., 1988), or quantitative genes (AVRDC, 1980; Leabwon and
Oupadissakoon, 1984). While PM resistance is controlled by either a
single dominant gene (Chaitieng, 2002a; Gawande and Patil,
2003; Khajudparn et
al., 2007) and 2 dominant genes (Reddy, 1994; Reddy, 2009) or quantitative genes (Young et al., 1993; Chaitieng et al.,
2002b; Kasettranan et al., 2010; Chankaew et al., 2013). Interestingly, Khajudparn et
al. (2007) reported that resistance to PM in each 3
resistant mungbean line, V4718, V4758 and V4785 from the AVRDC collection are
controlled by a single dominant gene with non-allelic interactions and showed
high resistance to the disease in Thailand. In addition, Chankaew et al. (2009) also indicated that resistance to CLS in V4718 is
controlled by a single dominant gene. Furthermore, our
previous studies have reported molecular markers linked to the CLS and PM in
each 2 resistant line. I85420 and I42PL229 ISSR and newly developed ISSR-anchored
resistance gene analog (ISSR-RGA) markers derived from the susceptible (CN72) and the resistant parent (V4718), respectively have been identified at
the distance of 4 and 9 centimorgan (cM),
respectively from a major QTL, qPMC72V18-1 of PM resistance (Oythip et al., 2017). I27R211 and I27R565 ISSR-RGA markers were linked to PM
resistance in V4785 (Oythip et al., Unpublished data). VR393 and CEDG084 SSR
markers were localized between the QTLs, qCLSC72V18 and qPMC72V18
conferring CLS and PM resistance in V4718, respectively (Arsakit et al., 2017). Thus,
these powerful markers can be immediately used to pyramid these resistance
genes into a recommended variety for durable resistance to CLS and PM through
marker assisted selection (MAS). In
addition, MAS for pyramiding desired genes along with background selection is
helpful in minimizing unlinked regions that negatively affect crop performance from
the donor segment and recovering the recurrent parent genome (RPG) within the early backcross (BC) generations (Hasan et al., 2015).
In mungbean, pyramiding desired CLS and PM genes with MAS have not been
successful. Therefore, this study attempt to successfully transfer and pyramid
the CLS resistance gene and 2 PM resistance genes from 2 resistant lines, V4718
and V4785 into suitable recurrent parents derived from diversity estimation
using marker assisted backcross breeding (MABB). The end products of a gene pyramiding
program are the resistant mungbean individuals having all 3 resistance genes
along with resembling the recurrent parent.          

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