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Modern Biotechnology as an Integral Supplement to Conventional Plant Breeding

The Prospects and Challenges

USDA–ARS, Northern Crop Science Laboratory, Fargo, ND 58105

View larger version (74K): [in a new window]   Fig. 1. Effects of Ph1 on chromosome pairing at meiosis in bread wheat haploids (2n = 3x = 21; ABD genomes). Pollen mother cells (PMCs) with and without Ph1 are shown. (A) PMC with Ph1 showing 1 rod II and 19 I. Such a superficial pairing in the presence of Ph1 has been called "chromosome dating." (B) PMC of a ph1b-haploid showing 6 II (3 ring and 3 rod bivalents) + 9 I. Note extensive homoeologous pairing because of the absence of Ph1. Interestingly, one gene can make such a difference. (Fig. 1B from Jauhar et al., 1991).

View larger version (56K): [in a new window]   Fig. 2. Durum wheat x Th. bessarabicum hybrid, and its chromosome pairing in the presence and absence of Ph1. (A) Spikes of parental species—durum wheat (left), Th. bessarabicum (right)—and their intergeneric hybrid (center). (B) PMC of the triploid intergeneric hybrid (2n = 3x = 21; ABJ genomes) with Ph1 showing 21 I. Note complete absence of pairing because of the presence of Ph1. (C) PMC of the intergeneric hybrid durum Langdon (LDN) disomic substitution 5D (5B) x Th. bessarabicum, showing 2 III [one V-shaped (arrow) and one frying pan–shaped (arrowhead)] + 4 II + 7 I. Note extensive homoeologous pairing, a welcome feature from the breeding standpoint. Some pairing takes place between the wheat and grass chromosomes (see Fig. 2D). (D) Same hybrid as in (C) with meiotic chromosomes after fluorescent genomic in situ hybridization when the durum wheat A-genome (colored green) was probed with Triticum urartu DNA labeled with FITC, the J-genome was probed with Th. bessarabicum DNA labeled with Rhodamine (colored red), and the remaining chromosomes counterstained with DAPI (colored blue) belong to the B-genome with one D-genome chromosome from 5D. Note wheat–grass pairing (A-J pairing).

View larger version (37K): [in a new window]   Fig. 3. Intergeneric hybrid (2n = 5x = 35; ABDPP genomes) between bread wheat (2n = 6x = 42; AABBDD) wheatgrass, Agropyron cristatum (2n = 4x = 28; PPPP), and genotype-induced homoeologous chromosome pairing. (A) Bread wheat var. Fukuhokomugi (left), crested wheatgrass (right), and their hybrid (center). Note the intermediate phenotype of the hybrid. (B) Meiotic metaphase in the hybrid showing 1 III + 10 II + 12 I. The genotype of the grass parent induces higher pairing than is expected based on chromosome homology.

View larger version (32K): [in a new window]   Fig. 4. Intergeneric F1 hybrids between bread wheat cultivar Fukuhokomugi (Fuko) and decaploid tall wheatgrass, Thinopyrum ponticum var., ‘Alkar’ (2n = 10x = 70). This hybridization offers prospects for breeding "perennial" wheat. (A) Seeds of the female parent Fuko (top row), intergeneric F1 hybrid (middle row), and the male parent Alkar (bottom row). Note large size of the seeds (with husk) of the hybrid. (B) Seeds of Fuko (top row), dehusked seeds of intergeneric F1 hybrid (middle row), and dehusked seeds of the grass parent Alkar (bottom row). (From Jauhar, 1995).

View larger version (24K): [in a new window]   Fig. 5. Schematic representation of the steps of evolution of "plant breeding." With the availability of more sophisticated tools, the art of plant breeding became science-based technology, and later led to the dawn of molecular plant breeding.