Ploidy analysis to facilitate banana improvement

Banana breeding involves interploidy crosses whereby 3x landraces and 4x hybrids are crossed with 2x males.  The progeny obtained range from euploid (2x, 3x, 4x) to aneuploid and undesirable hyperploids.  Screening the progeny for the exact ploidy is essential in selecting secondary triploids desired for their sterility, preferred, in edible bananas. Similarly, the diploid hybrids are selected for further improvement of the male stock. Flow cytometry (Fig.1) provides a more rapid and reliable way of ascertaining ploidy in Musa than conventional methodologies (chromosome counting and leaf morphology).  We screened over 1000 highland banana hybrids and found that 4x by 2x crosses yielded 88% triploids(3x), 8% diploids (2x) and 4% tetraploids (4x) hybrids, while 3x by 2x crosses yielded 99% tetraploids and 1% diploids. Selection was done when the hybrids were still in tissue culture. This early screening saved time, space and labour costs that would have been used if the hybrids were to be grown in the field prior to selection.

Development of a marker for parthenocarpy 

Three dominant genes (P1, P2, and P3) are reported to be involved in the control of fruit parthenocarpy in crosses between wild and cultivated diploid bananas. The seeded non-edible Calcutta 4 lacks the P1 gene and is homozygous for P2 and P3.  A segregating population was developed at KARI by crossing Calcutta 4 (seeded wild diploid) and Pisang Lilin (non-seeded edible diploid homozygous for P1). 250 plants were generated via embryo rescue from over 5000 seeds harvested from 7 different crosses. The behaviour of the unpollinated flowers of the two parental clones followed two developmental sequences:

Parthenocarpic: in Pisang Lilin, the ovaries swelled and become filled with edible pulp to give mature parthenocarpic fruits in about three months.

Persistent: in Calcutta 4, the ovaries swelled to some extent, persisted for 2 months although the ovules degenerated.  Finally no pulp was formed and the fruits shrivelled, rotten and eventually dried up a shown in figure 2.

We have done DNA extraction from these plants, run preliminary optimization experiments to select suitable primers and acquired capacity to carry out AFLP. DNA fingerprinting from leaf tissues is ongoing while fruit size, weight and segregation for parthenocarpy and other pulp traits will be scored at fruit maturity of the hybrids. cDNA from the developing fruits will be analysed using AFLP methodology and the banding patterns compared with other tissues to map and possibly isolate the parthenocarpy gene P1. A molecular marker for the P1 gene can enable the elimination of seeded non-edible hybrids at an early stage when the plants are still in the screen house. This will reduce the time, costs and space in selecting edible bananas for release to farmers.