Hybrid dwarfness in wheat

Abstract

Hybrid dwarfness is the phenomenon that after crossing of normal genotypes dwarfs are obtained in the Fl or not before the F2 generation . The literature on hybrid dwarfness in wheat is critically discussed . A new hypothesis on its genetic basis is given, taking MCMILLAN'S (1937) as a starting point . Dwarfness is assumed to be determined by the additive interaction of three genes DI , D2 and D3 , differing in dominance relations and in quantitative contribution to the dwarf phenotype .Three dwarf types are described. Type 1-dwarfs are dwarf during their whole life cycle and normally do not produce seeds . Type 2-dwarfs start as normal seedlings, become dwarfs while tillering and die dwarfs ; some produce seeds, others do not . Type 3-dwarfs emerge as normal seedlings, become dwarfs during the tillering stage, but after some time they start to shoot and develop into nearly or even completely normal plants ; in the F2 the proportion of dwarfs decreases during the growing season . The occurrence and genetic basis of the three dwarf types is discussed .On the basis of their genotype 315 varieties and lines are divided into six genotypeclasses . Over 1000 intra-and inter-class crosses were made and F,, F 2 , F3 , and BC studied . Also some triple crosses and crosses with pure breeding dwarfs were investigated . In general the results obtained fit the hypothesis . Unstable ratios involving type 3-dwarfs are discussed separately .Linkage of the dwarf gene D2 and the necrosis gene Nee (both on chromosome 2B (XIII) was apparent from F 2-data and from results of a triple cross in which both forms of hybrid weakness occurred . Crossing-over between D2 and Ne e is calculated to be 34% .Methods are outlined to use hybrid dwarfness in a wheat breeding programme . The possible incompleteness of the three-gene hypothesis and the variability of dwarfness are discussed and finally some suggestions are made for future research .

The location of the photoperiod gene, Ppd2 and an additional genetic factor for ear-emergence time on chromosome 2B of wheat

“…These workers, from the laboratory which originally developed the Marquis substitution lines, suspected that the donor parent was a hybrid between Thatcher and Marquis, rather than Marquis itself. Thus, it would appear that CS (Marquis 2B) is possibly CS(Thatcher 2B), since Thatcher is known to carry the same resistance gene as Marquis (Mcintosh, 1977) but lacks D2 (Hermesen, 1967).…”

“…An additional gene difference was reported between Chinese Spring and Marquis chromosome 2B, the D2 gene for hybrid dwarfing (Hermesen, 1967). This is one of a set of complementary dominant genes Di to D4 which produces dwarf phenotypes in specific combinations e.g., D1D2D3.…”

“…This is one of a set of complementary dominant genes Di to D4 which produces dwarf phenotypes in specific combinations e.g., D1D2D3. Chinese Spring has the recessive complement of all four dwarfing genes did2d3d4 (Hermesen, 1967;Worland and Law, 1980). McIntosh and Baker (1969) reported the location of D2 as 422 cross-over units from Sr9.…”

The Control of Growth Habit of Marquillo × Kenya Farmer Wheat Dwarf I by Temperature

“…The temperature sensitivity of the MqlxKF I hybrids was demonstrated by the chanige from the normal (2), to the "grass-clump" dwarf (2,6,8,9), the late heading "grass-clump" dwarf (6,8) and the "lethal" (6,8,9) morphological forms as the 160 phase of the temperature regime was lengthened.…”

“…Hyrbrids from crosses of many normal wheat varieties exhibit limited leaf elongation, profuse tillering and low fertility under field conditions (3,6,8) and have been termed "grass-clump" dwarfs.…”