Genetic diversity of cultivated crops and in situ conservation of genetic resources. – Botanica Lithuanica, Suppl. 2: 15–30
All crops evolved from wild species as a result of human activity at specific locations. Therefore, crop domestication consisted of the interaction between plants, their environment, and human beings. These interactions occurred from the early selection by humans of wild ancestors of current crops, and have remained throughout the cultivation of landraces by farmers, or the development of new cultivars by modern plant breeders. At the beginning of agriculture, these interactions took place in the centre of origin of each specific crop, and after that extended to the centres of diversity. Nowadays, it may be seen in farmers’ fields and it breeding stations and testing sites for cultivar development. Total genetic diversity in a crop species depends on the frequency of alleles controlling morphological and physiological characters selected during human domestication. For example, genes controlling the growth cycle (e.g. time of sowing or date of harvest) influence the evolution and domestication of most popular annual seed crops, and determine their rate and scale of adaptation. Thus, population structure of crop species arose from their original diversity patterns that could have been affected by the length of their life cycle. Consequently, the specific level of genetic variability observed in a defined crop population resulted from joint effects of mutation, migration, recombination, selection (both natural and artificial), and random genetic drift (i.e., changes due to change in small populations). Gene mutation, and gene flow via migration and recombination enhanced variability, whereas selection and genetic drift often tended to reduce this variability. Changes in gene frequency of favourable alleles, which increased crop fitness to the human-made environment for agriculture, were the driving force during the evolution of crop species. Owing to some morpho-physiological changes, the survival of some crop species depends today on human agriculture. The domestication and further evolution of some crops were also affected by isolating mechanisms, such as the natural pollinator(s), geographical barriers, self-incompatibility, sterility, and vegetative propagation. A dynamic system for preservation of plant genetic resources, such as in situ conservation (ISC), should be considered for crop species and their life relatives, because it allows the competition of crop population with other indigenous plants in their permanent or original eco-niches. Likewise, ISC permits interaction between the crop, its wild relatives, and in some sites their co-evolution with the pathogens affecting each species. Although many crops may be chosen for ISC, available resources have become the limiting factor for such a task. The most promising candidates for ISC are vegetatively propagated perennial crops, and those with recalcitrant seeds not amenable for ex situ gene bank conservation. Analysis of the pattern of allelic diversity in each crop and its wild relatives, may allow the development of a comprehensive ISC strategy for the preservation of plant genetic resources.
Keywords: cultivated crops, Musa, Solanum, Lycopersicon, Chenopodium, Hordeum, Vaccinium, Pruinus, Festuca, in situ conservation, genetic diversity.