Annona Species Monograph.pdf - Crops for the Future

Chapter 9. Genetic Improvement
9.1 Introduction
A. C. de Q. Pinto and S.R.M. de Andrade
Although restricted to only a few species, principally cherimoya,
improvement programmes of annonas have made great contributions by
producing important cultivars with good yields and fruit quality, and that
more closely meet consumers' demands. The development of new cultivars is
not a simple task. In many cases the major constraints are agronomic, as well
as the lack of prolific cultivars to start improving.
9.2 Cytogenetics and genetic aspects
The chromosome numbers of cherimoya, custard apple, soursop and sugar
apple are 2n = 14 to 16 (Nakasone and Paull, 1998; George and Nissen,
1992; Koesriharti, 1992). Although there is some variation in chromosome
number, they are all diploids, 2n = 2x. The chromosome number of wild
soursop is not reported. This slight variation in chromosome number may
explain the ease or difficulty of interspecific hybridisation and grafting, and
warrants further work to determine if intra-specific variation also exists.
Some related species, e.g., A. glabra, are known to be tetraploid (Kessler,
1993, cited by Scheldeman, 2002).
Generally, cross-pollination between annonas is conducted primarily to
determine compatibility for increasing fruit set (Nakasone and Paull, 1998)
and occasionally for new hybrid development. Samuel et al. (1991, cited by
Nakasone and Paull, 1998) commented that crosses among soursop and other
annonas, such as cherimoya, ilama, custard apple or sugar apple, have not
been successful. This may reflect the genetic distance between soursop and
the others. However, there is a dearth of information on species'
relationships, so this is a topic that needs to be researched.
The extensive morphological diversity, much of it genetically based, within
all Annona species (Page, 1984) not only offers great potential for breeding,
but also lowers the possibility of easily selecting a cultivar with all the
possible desirable characters. Considerable variation exists among cultivars
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