Читать книгу Principles of Plant Genetics and Breeding - George Acquaah - Страница 294

Typically, the generation of a maize × Tripsacum hybrid is accomplished by covering the developing maize ear with a bag to prevent cross‐contamination with corn pollen. Within a few days, ample silks are available on the ears and the silks are cut back to approximate 10–15 cm in length. Tripsacum pollen is gathered and dusted onto the maize ear. Typically, pollen from tetraploid Tripsacum (2n = 4x = 72) provides superior seed set, but occasionally, a diploid Tripsacum genotype (2n = 2x = 36) can be identified which also conditions superior seed development. When an appropriate maize genotype is utilized, embryo rescue techniques are not necessary. Experience has shown that a “Supergold popcorn” accession (PI222648) available from the USDA‐ARS Plant Introduction Station, Ames, IA, gives abundant and viable F₁ seeds (Kindiger and Beckett 1992) (Figure B6.2). Experimentation with other maize germplasm can provide similar if not superior results.

Figure B6.2 Hybrid seed set utilizing Ladyfinger popcorn as the maternal parent when pollinated by tetraploid T. dactyloides. Over 100 F₁ seeds can be readily obtained when an appropriate maize parent is utilized in the cross.

The F₁ hybrids are completely pollen sterile and microsporogenesis is associated with a varying array of meiotic anomalies (Kindiger 1993) and vary in seed fertility from completely sterile to highly seed fertile (Harlan and de Wet 1977). To date, all seed fertile hybrids generated from tetraploid Tripsacum dactyloides resources exhibit some level of apomictic expression, which following backcrossing with maize, is often lost. The most common or sexual pathway of genomic change in a series of maize‐Tripsacum backcross hybrids has been clearly described by Harlan and de Wet (1977). Comparative genetics and other approaches that may result in the transfer of Tripsacum traits to maize, including apomixis are described below.