ECOL3453 - Crop Ecology

Semester 2  2004-2005

Lecture 5 - Origins of Agriculture and Crop Plants 4

Crops are generally differentiated geographically, by past selection for environmental adaptation and use, often showing recognizable racial differentiation.

The degree of differentiation varies but rarely approaches the level of reproductive isolation that would imply incipient speciation. Speciation in cultivation would be surprising with man's habit of carrying his crop with him and the certainty of gene-flow between local races and between crops and weeds, a feature of crop evolution.

Geographical spread has been accompanied by genetical differentiation into locally adapted populations among which more or less distinctive races can often be recognised. Adaptation is one cause but there is also genetic drift and the founder effect. For some crops, e.g. oil palm, rubber and neem, we know that large poulations have been founded from very few individuals.

It seems very likely that the early movement of crops was often on a narrow genetic base, successive movements and adaptations, narrowing it still further. The joint effects of small founder populations and local adaptative pressures must tend to produce both geographical differentiation and tolerance of a degree of inbreeding.

It should be noted that trends towards increased inbreeding were often coupled with episodes of outcrossing. E.g. many disease resistances have been deliberately introgressed from wild relatives.

Crop evolution has a recurrent pattern of geographical spread followed by local adaptation and enhanced inbreeding, alternating with outcrossing episodes. Outcrossing would widen the genetic base and no doubt usually depress short-term adaptation but advance the possibilities of later advance.

Polyploidy

Most crop evolution has depended on gene-substitution at the diploid level, but polyploidy has also been important. Autopolyploidy is a feature of crops grown for vegetative products in which seed fertility is not important or may be positively disfavoured, e.g. bananas. It is evident that there has been selection for polyploidy related to vegetative vigour.

Seed production is critical in many allopolyploid crops, e.g. oats, wheats, cotton, coffee. In these, regular homologous pairing has widened the genetic possibilities without incurring impaired fertility. In some, the polyploidy preceeded domestication, e.g. tetraploid wheats and cottons; in others, it was concurrent with or followed domestication, e.g. Brassica crops, tobacco.

Although there are highly successful, seed-fertile, allopolyploid crops, it must not be presumed that new allopolyploids are automatically fertile from the start. They are often not since considerable genetic adjustment is necessary. But, allopolyploids seem to have a readier capacity for such adjustment than autotetraploids.

Another important feature of polyploids is that polyploids of outbred diploids frequently display some disruption of self-incompatibility, e.g. potatoes, or sexual differentiation, e.g. Cannabis. Self-pollination is thus possible in plants that, by reason of polyploidy, are better able to tolerate the consequences than the diploids from which they came. Therefore, polyploidy may have been a way of escaping outbreeding and an aid to adaptation in small, isolated populations.

Thus, the significance of polyploidy is varied and complex. Vegetative vigour, permanent hybridity, enhanced inbreeding and seed sterility have all been advantageous in crop evolution. But, polyploidy is only a supplement to gene-substitution as an evolutionary mechanism, not a substitute. Successful polyplois have to adapt at the polyploid level !

Breeding Systems

Most of the features of crop evolution mentioned so far are independent of the mating systems of the crops concerned. Charles Darwin was the first to make the distiction between outbreeders - species with various mechanisms promoting cross-pollination and showing inbreeding depression - and inbreeders - self-pollinators, vigorous over generations of inbreeding.

Most plants are outbreeders, promoting and crossing by a considerable variety of genetical and morphogenetic mechanisms (Table 2).

1. Monoecious

2. Flowers monoclinous

3. Self-pollinated --------------------------------- lettuce, groundnut, soybean, tomato
3. Cross-pollinated

4. Self-compatible

5. Wind-pollinated ---------------------- sugarcane
5. Insect-pollinated --------------------- avocado, onion, carrot

4. Self-incompatible

5. Homomorphic

6. Sporophytic ---------------------- 2x Brassica crops, radish
6. Gametophytic -------------------- pineapple, 2x potatoes
6. Other --------------------------------- kola, cacao

5. Heteromorphic -----------------------sweet potato

2. Flowers diclinous ------------------------------- mango, cucurbits, yams, maize, banana

1. Dioecious ----------------------------------------------- papaya, fig, hemp

Populations are variable and carry a load of deleterious recessive genes that are responsible for the inbreeding depression. Individuals in outbred populations are highly heterozygous. The population carries a segregational load due to the occurence of unfit individuals, the products of chance concurrence of deleterious recessive alleles; but it gains as a whole from a high recombination rate, evolutionary flexibility and the opportunity to exploit favourable recombinants.

Inbreeders have achieved a different balance. Deleterious rcessives are rare, the segregational load is trivial, prolonged inbreeding is tolerated, individuals tend to homozygosity, and the population tends to consist of a mixture of inbred lines. Local adaptation is high but at the expense of long-term flexibility to respond to change.

In practice, both in wild and crop plants, inbreeding is rarely perfect and a small percentage of cross pollination may be very important in maintaining recombination, heterozygosity and adaptability. Some inbreeders, although basically inbred in that they tolerate selfing, are sufficiently outcrossed to be classified as in-outbreeders.

The relationship between life cycle and breeding system in crop plants is shown in Table 3. This shows that the majority of perennials are outbreeders and that annuals are more likely to be inbreeders.

Summary

The mechanisms leading to crop evolution are the same as those of natural evolution. They include geographical-ecological differentiation of populations under selection for reproductive fitness; gene substitution processes coupled with recurrent hybridisation are fundamental, these are often supplemented by polyploidy. Selection in crops is, and has been, a combination of natural and human processes.