MOLECULAR ASPECTS OF HETEROSIS IN PLANTS

Since Shull's original description of heterosis, breeders have made wi de use of this phenomenon. However, while agronomists have been utiliz ing heterosis as a means of improving crop productivity, the biologica l basis of heterosis remains unkown. It is generally believed that our understanding of heterosis will enhance our ability to form new genot ypes which may be used directly as F-1 hybrids or form the basis for f uture selection programmes. While the original concept of heterosis re sulted from studies at the phenotypic morpohological level, they were soon followed by biochemical data with the advent of electrophoresis a nd the consequent ease of accumulation of data related to isozyme vari ability. However, the large number of restriction fragment length poly morphisms from more recent studies has allowed the development of link age maps with a high degree of resolution useful in locating and manip ulating quantitative trait loci (QTL). When substantial numbers of suc h neutral markers were used to measure genetic distance in large numbe rs of maize inbreds, very significant correlations were recorded betwe en parental genetic distance and hybrid performance. Through the same approach, a relatively small number of QTLs dispersed through the maiz e genome were identified which show clear overdominance expression con trolling heterosis. The hypothesis was made that some QTLs could code for regulatory proteins since these proteins are able to control a vas t array of other structural genes, the products of which are necessary for the expression of complicated characters such as yield and hetero sis for yield. The few such proteins identified thus far are all multi meric proteins with the heteropolymers exhibiting significantly differ ent activities in comparison with the homopolymers, that is in complia nce with the clear overdominance manifestation of the few QTLs analyze d so far. In addition, parameters derived from the Variability of geno me expression assessed through studies of polymorphisms in the amounts of individual proteins or mRNAs show numerous significant correlation s between these indices and hybrid vigor. These correlations supported the conclusion that QTLs could be loci controlling the amount of mRNA s or proteins synthesized from a number of structural genes, and stres s the significance of both the regulatory proteins (and their encoding genes) and the structural genes, being regulated, in manifestation of complicated characters, such as heterosis.