Читать книгу Cucurbits - James R. Myers - Страница 38

Two genes of watermelon have been reported to govern anthracnose resistance, Ar-1 for race 1 and Ar-2-1 for race 2 resistance. Alleles db and Fo-1 provide resistance to gummy stem blight and race 1 of Fusarium wilt, respectively. However, gummy stem blight resistance appears to be due to more than just a single gene. Susceptibility to powdery mildew is governed by pm. Resistance to race 2 powdery mildew from PI 270545 is controlled by at least two genes. Resistance to the watermelon strain of papaya ringspot virus (PRSV-W) is controlled by a single recessive gene, prv. A moderate level of resistance to zucchini yellow mosaic virus was conferred by a single recessive gene zym-FL. A high level of resistance to the Florida strain of zucchini yellow mosaic virus was controlled by a single recessive gene, zym-FL-2; not the same as zym-FL. Resistance to the China strain of zucchini yellow mosaic virus was controlled by a single recessive gene zym-CH. A single dominant allele, Zym, confers resistance to zucchini yellow mosaic virus. Insect-resistance genes for watermelon include Af (red pumpkin beetle resistance) and Fwr (fruit fly resistance).

Watermelon plants with short vines can be bred if they are homozygous recessive at the dw-2 locus or for one of the two alleles for dwarf plant habit at the dw-1 locus. Both genes make a super dwarf that has been used to develop cultivars for patio and container production. Branching at the lower nodes of the main stem is reduced by allele bl.

Andromonoecy is recessive to monoecy and conditioned by gene a. Two alleles are known that produce male sterility, ms and gms, the latter associated with glabrous foliage.

When plants are homozygous recessive for the e (explosive rind) allele, the fruit is tender (not tough rind, but can be thin or thick rind), bursting when cut. The f gene determines furrowed fruit surface. The incompletely dominant allele O governs elongate versus spherical fruit shape. The su allele suppresses fruit bitterness.

Dark green skin colour (G-1 and G-2) is dominant and other alleles at the g-1 locus determine the degree of colour and striping: G (medium or dark solid green); gW (wide stripe); gM (medium stripe); gN (narrow stripe); and g (solid light green or grey). The dominance series is G > gW > gM > gN > g. Greenish mottling of the exocarp is produced by the m allele, and pencilled lines by p. Golden mature fruit colour and chlorosis of older leaves is governed by gene go.

A gene with a dominant allele for white flesh (Wf) is epistatic to a gene for yellow flesh; the double recessive is red-fleshed. Allele C produces canary yellow flesh colour. The darkness of red colour in the flesh (and the amount of lycopene) is controlled by multiple alleles at the y locus. Scarlet red flesh (YScr) is dominant to coral red flesh (YCrl), orange flesh (yO) and salmon yellow flesh (y). The dominance series is YScr > YCrl > yO > y. The allele YScr is from ‘Dixielee’ and ‘Red-N-Sweet’; the allele YCrl is from ‘Angeleno’ (black-seeded); the allele yO is from ‘Tendersweet Orange Flesh’; and the allele y is from ‘Golden Honey’.

The interaction of alleles at several genes, including d (dotted seed coat), r (red), t (tan) and w (white seed coat), determine seed coat colour and pattern. Clump is RR TT ww; tan is RR tt WW; white is RR tt ww; green is rr TT WW; red is rr tt WW; and white with pink tip is rr tt ww. Seed size is controlled by several genes, including l (long) and s (short seed) genes, with s epistatic to l, and long recessive to medium or short. The phenotypes are LL SS for medium, ll SS for long, and LL ss or ll ss for short seed. The sources for genotypes are ll SS from ‘Peerless’, LL SS from ‘Klondike’, and LL ss from ‘Baby Delight’. A third gene, ts, produces tomato seed size, which is smaller than short seed. Also, tiny seed (Ti, dominant to short seed) from ‘Sweet Princess’ produces seeds that are between short and tomato seed sizes. Five genes for seed protein composition (Spr-1, Spr-2, Spr-3, Spr-4 and Spr-5) are included in the watermelon gene list.

Four linkage groups of 13 isozyme genes were reported by Navot and Zamir (1986). Since then, the Citrullus genome has been sequenced using the Chinese breeding line 97103 (Guo et al., 2013), and later using ‘Charleston Gray’.