Plantains and bananas are ranked as the fourth most important food commodity after rice, wheat and milk (Ortiz and Vuylsteke 1996; FAO 1999). Yet despite the importance of these crops for international trade and food security in tropical regions, little attention has been given to their genetic improvement in comparison to other major food crops. As a consequence, major advances in Musa productivity have traditionally relied on improvements in crop husbandry. More recently good progress has been reported in the genetic understanding (Ortiz 1995) and enhancement of Musa crops (Rowe and Rosales 1996; Vuylsteke et al. 1997). Nevertheless, there remains an enormous potential for increasing yields in Musa crops through genetic improvement and there is an urgent need to improve our understanding of the genomic structure and genetic relationship of the parental and progeny genotypes used in Musa breeding programs across the world. Microsatellite markers have proven useful for genetic analysis in a number of systems including Musa (Crouch et al. 1998; 1999). This study uses variable number of tandem repeats (VNTR) analysis of microsatellite loci to compare the genetic similarity of full-sib 2x and 4x hybrids, and their parental genotypes.

Analysis of breeding populations with microsatellite markers clearly holds considerable promise for improving the design and operating efficiency of Musa breeding schemes. In this study we have demonstrated the ability of microsatellite markers to detect very high levels of polymorphism in full-sib Musa breeding populations. Over three-quarters of the functional microsatellite markers detected polymorphisms within the breeding populations studied here.

Based on the data generated from this study, we propose that there is a significant frequency of duplicated loci in both Calcutta 4 (AA) and Obino l'Ewai (AAB). The duplication of genes (and also of entire chromosomes) has been reported in many crop species, where it is often indicative of evolutionary polyploidization. Cultivated and wild bananas have a basic chromosome set of x = 11 but these data support the hypothesis that this is a secondary haploid number perhaps derived from an x = 8 progenitor.

A high proportion of loci segregated within the full-sib tetraploid progeny population. This result confirms the high level of heterozygosity of both parental genotypes and the high rate of recombination during the formation of 2n (=3x) megaspores by Obino l'Ewai. Musa breeding strategies are commonly based on crosses between diploid and triploid accessions, which only yield small progeny populations. Nevertheless, these data demonstrate that even when moderate population sizes are used, the level of segregation and recombination observed is sufficient to facilitate efficient introgression of important agronomic characters from exotic germplasm.

Musa improvement programs across the world have concentrated on the generation of diploid and tetraploid hybrids such as those studied here. However, Musa breeding at IITA is currently focused on crossing diploid and tetraploid hybrids in order to generate secondary triploid Musa hybrids suitable for cultivar release. Based on this molecular marker data generated in this study, it is possible to identify two diploid hybrids, which are highly genetically divergent from two full-sib tetraploid hybrids. Crosses between these two groups would be an appropriate strategy for capturing genetic diversity in the resultant secondary triploid progeny. However, it is likely that specific genetic factors (as opposed to broad genetic diversity) also have a major contribution to high yield in polyploid Musa hybrids (Ortiz 1997). On this basis two strategies will be important in breeding secondary triploid Musa hybrids: the generation of breeding populations presenting diverse heterotic groups (at the 2x and 4x levels), and, the utilisation of recombination to form genetic structures more favourable than those already present in plantain landraces.

Finally, we report a high multiplex ratio for microsatellite marker analysis of diploid and polyploid genotypes plus their hybrids, which may result from the presence of genomic duplications and homoeologous loci. This complex genomic structure greatly complicates interpretation of data from molecular markers. Thus, for many applications, pre-screening will be necessary to define a subset of markers which generate more simple datasets. Only in this way will it be possible to use microsatellite markers to identify homozygotes and heterozygotes in Musa breeding populations. This type of information will become increasingly important, as for example, attempts are made to accurately define combining ability of potential parental genotypes in triploid Musa breeding, and to apply marker assisted selection in progeny populations.

Crouch, H.K., Crouch, J.H., Jarret, R.L., Cregan, P.B. and Ortiz, R. (1998). Segregation of microsatellite loci from haploid and diploid gametes in Musa. Crop Science 38:211-217.

Crouch, J.H., Crouch, H.K., Constandt, H., Van Gysel, A., Breyne, P., Van Montagu, M., Jarret, R.L. and Ortiz, R. (1999). Comparison of PCR-based molecular marker analyses of Musa breeding populations. Molecular Breeding 5:233-244.

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