Cytological studies of diploid and tetraploidTheobroma cacao

Abstract

The chromosomes of diploid and established artificial autotetraploidT. cacao were examined with particular attention to their shapes and relative sizes at mitotic metaphase, as well as their association during metaphase of male meiosis. Bivalent formation was observed in the diploid, and although there was low apparent chiasma frequency, disjunction was regular. In the two established autotetraploids studied, there was a high frequency of bivalents as compared with trivalents and quadrivalents. Pollen fertility as measured by its germination on agar was between 38 and 64 percent (diploid 90–98.5 percent). Fruit set was between 11.5 and 19.1 percent (diploid 74 percent), and mature bean number was approximately half the ovule number counted (diploid 90 percent). Seed germination was, however, never less than 77 percent at any particular sowing and almost all the seedlings had 4x chromosome number. It is suggested that an improvement in the yield capacity of the autotetraploid might be possible by crossing C₁ tetraploid individuals selected on the basis of the frequency of bivalent formation, as well as uniformity in pollen size and fertility.