Читать книгу Cucurbits - James R. Myers - Страница 34

Genetic knowledge of cucurbits is behind that of maize, tomato and pea, despite the considerable natural genetic variation in many species of the Cucurbitaceae. The use of winter greenhouses, trellises and cages has reduced the space and labour requirements for crossing cucurbits for genetic studies. The establishment of the Cucurbit Genetics Cooperative, along with the publication of the Cucurbit Genetics Cooperative Report annually since 1978, has fostered communication among cucurbit researchers and stimulated more cytogenetic investigations.

Dominance relationships of genes in melon were first investigated by Sagaret in the mid-19th century. Mendelian inheritance in cucumber was reported in 1913, and many genes for each of the major cucurbit crops have since been identified. In 1976, a total of 170 individual genes were known for the Cucurbitaceae. Of these, 68 were for cucumber, 37 for melon, 30 for squash species, 25 for watermelon and ten for other genera. Numerous other genetic factors were known for cultivated cucurbits and used in breeding programmes, but were not included in the gene list because their inheritance was complex or unknown.

Many additional cucurbit genes and alleles have been identified in the intervening decades and, with the application of genomics to the various cucurbit species, the pace is accelerating. The Cucurbit Genetics Cooperative publishes gene lists for the major cucurbit crops and the earlier gene lists included 146 loci for cucumber (Wehner, 1993), 100 for melon (Pitrat, 1994a) and 81 for watermelon (Rhodes and Zhang, 1995). As of 2014, the number of genes published was 509, with 167 for cucumber, 160 for melon, 93 for squash species, 62 for watermelon and 27 for other genera. Thus, the number of known genes for the cucurbit crops has tripled in the past 40 years. Some of those genes code for isozymes. In addition, multiple alleles have been identified, e.g. YScr > YCrl > yO > y and G > gW > gM > gN > g loci in watermelon. It should also be mentioned that identification of a gene does not necessarily indicate specific knowledge about the location, structure, or sequence of that gene. In most cases, many of these genes do not yet have molecular markers associated with them.

Regarding gene nomenclature, it should be pointed out that most genes (though not those of enzymes) are recognized and named according to the discovery of an atypical expression of that gene, which is itself caused by a newly found allele at a previously unrecognized locus. The gene and atypical, or ‘mutant’, allele are given the same designation (e.g. gl is an allelic form of gene gl). The first letter of the symbol is in lower case if the atypical allele is recessive and uppercase if dominant. The designation for the normal allele of that gene is given as +, the gene name with a superscripted +, or the gene name with the first letter in the case (lower or uppercase) opposite that for the atypical allele. Continuing the above example for the glabrous (gl) gene, the common or normal allele can be designated as +, gl⁺ or Gl. In this volume, we use the superscript system for referring to common alleles.

When additional alleles are found and assigned to a gene, they are designated with different superscripts appended to the gene name. However, allelism testing runs far behind the discovery of genetic anomalies. When several alleles affect the same heritable trait, they are often treated as belonging to different genes until proven otherwise, although allelism tests are recommended before proposing another locus. This may be a problem for many alleles relating to disease resistance in cucurbits. More testing is needed to assign various alleles to their proper gene locus and to determine gene linkages.