Molecular aspects of heterosis in plants

Abstract

Since Shull's original description of heterosis, breeders have made wide use of this phenomenon. However, while agronomists have been utilizing heterosis as a means of improving crop productivity, the biological basis of heterosis remains unkown. It is generally believed that our understanding of heterosis will enhance our ability to form new genotypes which may be used directly as F1 hybrids or form the basis for future selection programmes. While the original concept of heterosis resulted from studies at the phenotypic morpohological level, they were soon followed by biochemical data with the advent of electrophoresis and the consequent ease of accumulation of data related to isozyme variability. However, the large number of restriction fragment length polymorphisms from more recent studies has allowed the development of linkage maps with a high degree of resolution useful in locating and manipulating quantitative trait loci (QTL). When substantial numbers of such neutral markers were used to measure genetic distance in large numbers of maize inbreds, very significant correlations were recorded between parental genetic distance and hybrid performance. Through the same approach, a relatively small number of QTLs dispersed through the maize genome were identified which show clear overdominance expression controlling heterosis.The hypothesis was made that some QTLs could code for regulatory proteins since these proteins are able to control a vast array of other structural genes, the products of which are necessary for the expression of complicated characters such as yield and heterosis for yield. The few such proteins identified thus far are all multimeric proteins with the heteropolymers exhibiting significantly different activities in comparison with the homopolymers, that is in compliance with the clear overdominance manifestation of the few QTLs analyzed so far.In addition, parameters derived from the variability of genome expression assessed through studies of polymorphisms in the amounts of individual proteins or mRNAs show numerous significant correlations between these indices and hybrid vigor. These correlations supported the conclusion that QTLs could be loci controlling the amount of mRNAs or proteins synthesized from a number of structural genes, and stress 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.