NEW DELHI, INDIA
Indian Agricultural Research Institute Analysis of genetic relationships in elite maize inbred lines and their relationship to heterotic grouping --Kumari, J, Gadag, RN In view of current emphasis on single cross hybrids in maize breeding strategy in India, we attempted comprehensive characterization and genetic analysis of maize inbred lines being used by a maize improvement unit. The ten parental lines (originating from four source populations) were characterized using eighteen easily identifiable morphological traits and molecular (SSR) markers. Very few morphological markers were found to be unique to the parental lines, implying restricted utility of morphological data in accurately differentiating the inbred lines. Polymorphism at the genomic level in these lines was analyzed using 27 polymorphic SSR markers, and it revealed eight unique or rare alleles specific to four inbred lines (DMB 101, 105, 106 and 109). This can help in identification and differentiation of inbred lines by multiplexing SSR markers having different size-ranges. Three inbred lines, DMB 102, DMB 104 and DMB 106, were found to be highly homozygous on the basis of molecular marker analysis also. Analysis of genetic relationship was attempted using the data sets generated by morphological and SSR markers individually, and in combination by cluster analysis. The ten inbred lines were grouped into three clusters (Fig 1).

DMB 107 and DMB 108 (derived from A-64 source population) clustered together. Also, the similarity of DMB 110 and DMB 104, which shared a common source population, was implied. However, many discrepancies were noticed in the general clustering patterns (on the basis of morphological and SSR markers individually or in combination). This could be possibly due to a limited number of traits, low SSR markers, multiple underlying genes for these traits and scoring errors. The broad base of the source population, as well as over-representation of a particular source population (for example, five of the ten genotypes were derived from A-64), might be the reason for the low level of conformity between genetic relationships vis a vispedigree information. It is not uncommon to expect inconsistency between SSR grouping and genetic background of corn inbreds, and this may be attributed to many factors (Yu et.al., Maydica 46:133-139, 2001).

Figure 1. Dendrogram depicting genetic relationships among selected inbred based on morphological and SSR data

Superior heterotic single cross combinations in comparison to PEHM-2 (the check with comparable maturity) were identified (DMB 101 x DMB 109, DMB 102 x DMB 103, DMB 102 x DMB 110, DMB 103 x DMB 104 and DMB 104 x DMB 110) on the basis of per se performance (Table 1). Of the five elite hybrid combinations identified over two locations, three crosses (highlighted) involved the parental line belonging to different clusters generated by the combined data of morphological and molecular markers. Hence, the clustering pattern can be taken as a general indicator for choosing potential heterotic combinations. The relationship between hybrid relatedness and/or pedigree information, in terms of heterosis performance, can be explained and interpreted (Smith and Smith, MNL 63:86-87, 1989; Smith et.al., Maydica 45:235-241, 2000)

The fact that all parental inbred lines involved in these crosses (except DMB 110) were derived from the same source population, A-64 (Table 2), gave further credence to the broad base of the source population.

Table 1. Particulars and performance of promising maize single cross hybrids
 
       
Days to 50%
   
Heterosis for Gr. yield/plant over PEHM-2 (%)

Tasseling

Silking
S. No Cross
Delhi
Karnal
Delhi
Karnal
Delhi
Karnal
1.  DMB 103 X 104
17.9
8.3
51
53
54
56
2.  DMB 101 X 109
17.9
4.2
49
50
52
53
3.  DMB 102 X 103
10.3
3.9
49
47
51
49
4.  DMB 104 X 110
7.7
2.5
53
52
56
54
5.  DMB 102 X 110
2.6
12.5
51
48
54
51

Table 2. Particulars of maize inbred lines involved in elite single cross hybrids
 
Inbred No. Pedigree Source Population
DMB 101 IPA 3-6-10-3-1-1-1-2-1-# A-64
DMB 102 IPA 3-f (-1) A-64
DMB 103 IPA 3-f (-2)  A-64
DMB 104 IPA 1-f-16-2-#-f-1 A-64
DMB 109 TCA 22-3-1-1-1-f-#-f-1 A-64
DMB 110 SC 7-2-1-1-7-1-1-1-1 Derived from MDR-1 X A-64

In the present investigation aimed at characterization of selected maize inbreds, SSR markers were instrumental in finer discrimination of inbred lines, as well as more precise analysis of homozygosity. Though differentiation of maize lines on the basis of morphological traits is presently contemplated, for finer discrimination between the parental lines, molecular markers like SSRs will be used. Some discrepancy in clustering could be accounted for by various factors. Molecular markers can serve as an invaluable aid for a variety of applications in maize breeding (Dudley et.al., Crop Sci. 31:718-723, 1991).
 
 

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