Predicting Heterosis Using Dna Markers In Plant And Animal Species

There is a long history of using breed and/or stain crosses in the pro- duction of plants and animals, mainly to take advantage of heterosis. The word "heterosis"was coined by Shull in 1914 to describe the increased vigor of crossbreeds relative to their parents, irrespective of the cause. In modern breeding of plants and animals, heterosis is most commonly referred to as the deviation between the cross and midparent means. The introduc- tion of hybrid seed corn in the l930s, followed by its phenomenal success in commercial production, promoted breeders to seriously consider cross- breeding. Since then, crossbreeding has become the predominant mating system in the commercial plant and animal industries. Two basic genetic mechanisms have been proposed to account for het- erosis: dominance and epistasis. The former explains heterosis by interac- tions among alleles at the same iocus while the latter attributes the cause of heterosis chiefly to interactions between different loci. In recent years, a great variety of polymorphism has been observed at phenotypic,biochemical, Wu Xiaolin: Predicting heterosis using DNA markers in plant and animal species and molecular level, which seems to be associated with increased vitality and adaptability in plant and animal populations. According to the theory of balanced polymorphism, when all polymorphic genotypes, no matter they result from chromosomal aberration or gene mutation, reach a state of bal.- ance, heterosis is due. In other words, if a heterozygote carrying a mutant gene surpasses a homozygote in adaptability, the superiority of heterozygosi- ty will remain in this population in a long period of time, however harmful or even detrimental this mutant gene may be. Thus, heterosis is defined as the relative superiority of heterozygotes over homozygotes in productive adaptability, which includes vitality, longevity, reproductivity, or combination of these factors from periods of ova to adults. There were also many occa- sions when significant interaction was found between heterosis and environ- mental effects, which explained at least partially how heterosis was pro- duced. According to the concept of genetic homeostasis, attributed to Lerner (1954), heterozygotes are likely to be better buffered than homozygotes a- gainst environmental variation. Consequently, heterozygotes might be expect- ed to display less variation, both within and between different environments. Ever since the first recognition of heterosis, breeders and scientists have been seeking efficient ways for predicting levels of heterosis. In early researches, the effect of crossbreeding were evaluated by practical experi- ments. Experiment station investigation into the effect of crossbreeding in animals began in the 1980s. However, since the number of available breeds of plants and animals, and the variety of alternative crossbreeding systems is so large, it is impractical to experimentally evaluate all possible crossbreeding systems. Moreover, meaningful comparison among crossbreeding systems requires an evaluation of all economically important traits and should include all purebred and multiplier populations necessary to support specific crossing systems for production of plants and animals. Understand- ing of the genetic basis of crossbreeding effects was enhanced by Hender- son (1948), who modified the models of Sprague and Tatum (1942) to esti- mate general, specific and maternal combining ability and sex條inked ef- fects. Further an...