| Summary: | With the exceptions of a small number of species, the genetic systems of tropical tree species suitable for planting in arid zones are scarcely known. A genetic system includes the species gene pool (with its development and activity in evolution and ecological genetics), the breeding unit (natural and artificial populations with characteristics gene frequencies) and the breeding system (mating or reproductive behaviour including techniques for the dispersal of gametes or seeds). The natural variation and taxonomy of selected tree species which have been studied by staff of the Commonwealth Forestry Institute in the field in Central America and Mexico are considered and the basis of the variation, as reflected by the type of breeding system employed, is analysed and discussed. The overall population structure as it is affected and constrained by a particular breeding system is considered; studies of pollen vectors, isozymes and other chemical evidence offer valuable techniques for elucidating such structures. Arid and semiarid lands (ASAL), particularly in developing countries, have special characteristics of location and accessibility, climate and soils, human social and economic structures, and products and services required. These impose constraints on selective breeding programmes and emphasize the need for simple population (provenance) testing. A wide range of species is available, including many nitrogen-fixing trees, and methods of evaluating these are discussed. Several internationally oriented institutions, including CFI, CSIRO, FAO/IBPGR and NAS, are conducting programmes of exploration, evaluation and genetic conservation of ASAL trees, and many national organizations are involved in all these stages for their indigenous species and with the coordinated international evaluation of exotic species. The special problems and techniques are described. Selective tree breeding has only been pursued seriously with species of Casuarina, Eucalyptus, Leucaena and Prosopis and there are constraints on the production and management of improved material in many developing countries. Nevertheless there are opportunities for socio-economic benefit from breeding of tree species for the arid and semiarid lands. © 1986.
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