Regions of genomic similarity among four 'Stiff Stalk' inbred lines as measured by multiple restriction enzymes in RFLP analysis
--B.E. Zehr and S. Wright

Multiple restriction enzymes have been used for each of 157 probes in RFLP analysis in order to reveal the extent of genomic similarity among four inbred lines derived from Iowa Stiff Stalk Synthetic (BSSS). Data for 142 probes were obtained using four restriction enzymes per probe (HindIII, EcoRI, EcoRV, SstI), while three enzymes were used for each of the 15 remaining probes. The extent of marker coverage per chromosome ranged from 26 probes (chromosome 1) to 9 probes (chromosome 10). The inbred lines used in this analysis were B14A, B37, B73, and B84; each line having proven to be of some historical significance in hybrid breeding, and each derived from either the initial cycle BSSS or an improved version.

Out of 157 probes total, 24 (15%) showed monomorphism among all four inbred lines for each restriction enzyme (Table 1). Thirteen of the 24 completely monomorphic probes were single banded, while the remainder identified multiple banding patterns with the primary band common (monomorphic) among all lines. Over 100 probes (64%) showed monomorphic expression for at least one restriction enzyme, while 56 probes (36%) identified no monomorphism across all four lines. (Table 1).

Table 1. Number of RFLP probes showing monomorphism among lines B14A, B37, B73, B84 for varying levels of restriction enzymes.
 

Monomorphism	Number of probes	% of total probes
For all 4 enzymes	24	15%
For at least 3 enzymes	36	23%
Monomorphism for at least 2 enzymes	59	38%
Monomorphism for at least 1 enzyme	101	64%
No monomorphism	56	36%

Chromosome numbers 1, 6 and 9 contained the most probes which were monomorphic for all restriction enzymes (Table 2), suggesting a greater degree of genomic conservation for these chromosomes among the four Stiff Stalk lines. Chromosomes 4 and 10 were not represented among probes showing monomorphism over all restriction enzymes. However, these chromosomes also contained the least number of probes; 10 probes on chromosome number 4, and 9 probes on chromosome number 10.

Table 2. Probes showing monomorphism among lines B14A, B37, B73, B84 for all restriction enzymes used in RFLP analysis.
 

Chrom. Arm	Probe	Chrom. Arm	Probe
1S	umc157	6S	bnl6.29
1L	adh1	6S	umc85
1L	bnl5.59	6L	umc21
1L	npi225	6L	umc65
1L	umc106
		7S	npi400
2L	umc98	7S	opaque2
		7L	bnl16.06
3L	umc02
3L	umc97	8L	umc93
		8L	umc120
5S	bnl6.25
5L	bnl5.71	9S	umc105
		9L	bnl14.28
		9L	bnl3.06
		9L	npi97
		9L	npi291

Genomic regions showing greatest conservation among lines were present both as isolated loci and as larger linkage groups. Figure 1 illustrates relative linkage arrangements and levels of monomorphism for probes on chromosome numbers 1 and 6. For chromosome 1, loci with high degrees of allelic conservation (monomorphic for all enzymes) were adjacent to regions with little or no monomorphism. In contrast, all highly monomorphic probes on chromosome 6 were adjacent, and located on the short arm and centromeric regions. However, marker saturation in these regions of chromosome 6 was somewhat limited.

These data are in agreement with that of Neuhausen (MNL 63:110) with regard to isolated regions of conservation among Stiff Stalk material on chromosome 1, and monomorphic banding patterns at most probes on chromosome 7 among inbred lines B73 and B84 (data not shown), both derived from later cycles of BSSS. Our data indicate a high degree of genomic conservation among both early cycle (B14A, B37) and later cycle (B73, B84) lines only at loci toward either end of chromosome 7. However, conservation among all lines could have been inferred for loci spread across this entire chromosome if ? 2 restriction enzymes had been used. This illustrates the value of utilizing multiple restriction enzymes for RFLP analysis when interpreting genomic similarity among individuals or lines.

Maize should be an excellent organism for studying long-term effects of selection with respect to genomic conservation. Each inbred line in this study represents an end point for separate selection experiments which have utilized the same genetic source material, and which have had similar goals. The rationale behind long-term recurrent selection in population improvement, like that ongoing for BSSS, should favor a degree of genomic conservation which would then be reflected through inbred development. We suggest that regions showing greatest degree of conservation among the lines in this study, as evidenced by monomorphic patterns viewed over multiple restriction enzymes for each RFLP probe, could be prime candidates to contain the genetic factors which have helped define 'Stiff Stalk' as an important heterotic group of hybrid corn.

Figure 1. Number of restriction enzymes (out of 4 total, or * = out of 3 total) showing monomorphism among inbred lines B14A, B37, B73, B84 for probes on chromosomes 1 and 6 (note: relative distances between markers are not proportionally accurate).

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