International Rice Research Institute, P. O. Box 933, Manila, Philippines
We studied association of some isozyme loci (viz. Amp-1, Amp-3, Est-2, Est-9, Pgi-1, Pgi-2) with quantitative traits (viz. grain yield per plant, number of spikelets per panicle, spikelet fertility, number of primary and secondary branches of panicle, plant height, tiller number, leaf width and leaf length) in F2 populations of two crosses: IR4683OA/IR9761-19-1R (cross 1) and IR58025A/IR54742-22-19-3R (cross 2) and the BC1F1 populations of two crosses, viz. IR54752A/IR64R (cross 3) and IR64/Dular (cross 4). Plants of each population were grouped
Table 1. Mean values of some agronomic traits of homozygote and better homozygotes for three isozyme loci in F2 generation of two rice crosses: IR4683OA/IR9761-19-1R and IR58025A/IR54742-22-19-3R
=====================================================================
Mean valuea
=======================================
Isozyme Agronomic traits Cross 1 Cross 2
Locus =================== ===================
Hetero. Homo. Diff. Hetero. Homo. Diff.
=====================================================================
Est-2 Number of spikelets per
panicle 72.8 79.8 -7.0** 100.8 115.9 -15.1*
Number of secondary
branches per panicle 12.6 13.9 -1.3** 15.8 18.1 -2.3*
Amp-3 Number of spikelets per
panicle 73.0 80.2 -7.2** 100.8 111.8 -11.0ns
Number of secondary
branches per panicle 12.7 13.9 -1.2** 15.9 17.9 -2.0ns
Pgi-2 Number of spikelets per
panicle 73.0 81.7 -8.7**
=====================================================================
a Difference between the mean of heterozygotes and better homozygotes of
cross 1 (IR4683OA/ IR9761-19-1R); cross 2 (IR58025A/IR54742-22-19-3R).* Significant at 5% level using t-test.
* Significant at 1% level using t-test.
ns Not significant.
according to allelic composition of each isozyme locus. For each trait of interest, mean performance of plants that were heterozygous at a given isozyme locus was compared with the performance of plants that were homozygous, using Student's t-test. In the case of the F2 population, the performance of the heterozygotes was compared to better performing homozygous class. For the BC populations, comparison were made between heterozygotes and the single homozygous class. A significant difference between the means of heterozygotes and homozygotes indicated that the isozyme locus was associated with a gene or block of genes which contributed to the quantitative trait.
Results (Tables I1and 2) showed that only three isozyme loci (viz., Est-2, Amp-3 and Pgi-2) located on chromosome 6, were consistently associated with two quantitative trait loci. Other quantitative traits did not show consistent results over crosses. Est-2 locus was associated with genes controlling number of spikelets per panicle and number of secondary branches of panicle in the four crosses. Amp-3 and Pgi-2 loci were associated with genes controlling number of spikelets per panicle and number of secondary branches of panicle in three crosses. Pham (1988) reported that Pgi-2 was a marker for spikelet fertility and plant height in a Japonica-Indica cross.
Above results suggest that isozyme loci can be used as markers of quantitative traits. It is however , useful if parents of crosses are polymorphic at many isozyme loci. In order to reduce the confounding effect of genotype/environment inter-
Table 2. Mean values of some agronomic traits of heterozygotes and homozygotes for some isozyme loci in BC1F1 population of two rice crosses: IR54752A/IR64R and IR64/Dular.
=====================================================================
Mean Valuea
======================================
Isozyme Agronomic traits Cross 3 Cross 4
Locus ======================================
Hetero. Homo. Diff. Hetero Homo. Diff.
=====================================================================
Est-2 Number of spikelets per
panicle 97.6 108.8 -11.2* 91.9 80.5 11.4*
Number of secondary
branches per panicle 18.5 20.4 - 1.9* 19.4 17.2 2.2*
Amp-3 Number of spikelets per
panicle 98.9 107.3 - 8.4* 91.5 80.6 11.0*
Pgi-2 Number of spikelets per
panicle 98.0 107.3 - 9.3* 91.9 79.2 12.7*
======================================================================
a Difference between the mean of heterozygotes and homozygotes of cross 3
(IR54752A/IR64R and cross 4 (IR64/Dular).* Significant at 5% level using t-test.
action in associating quantitative traits with isozyme markers, it is important to use recombinant inbred or doubled haploid progenies of a cross so that these progenies can be replicated over environments.
We thank Dr. D.S. Brar and Dr. D.P. Baldos for their comments and suggestions regarding the experimental procedures.
References
Pham, J. L. 1988. Isozyme markers for plant height and spikelet fertility in rice. RGN 5: 108- 109.