PISUM Genetics__________________________2003—Volume 35_________________Brief Communications
Identification of recessive er gene for powdery mildew resistance in a landrace of Pisum sativum
Sharma, B. . Div. of Gen., Indian Agric. Res. Inst.
A sample of seed was collected by the author in January 2000 from the field of a farmer in Dharwad distric t of Karnataka in South India. The plant had features typical of a primitive cultivar, such as tall and thin stems, red flowers (A), small leaves, flowers and pods, small seeds with black mottling on green testa, and dehiscent pods. The seeds are round (R) with mild rectangular configuration. The leaflets have red color at the margin, and the stipules lack an anthocyanin ring at the base (presumably a result of the d allele). It flowers in about 45 days under Delhi conditions.
A germplasm accession identified as P. arvense (P 1890) with similar (but not identical) features was received from the National Bureau of Plant Genetic Resources, Delhi, as NIC 11184. It was collected as a landrace from village Kanwa in the Kota district of Rajasthan in western India in 1992 (collector's record NKD/TSA/1275 dated 27.1.1992).
Both these genotypes turned out to be resistant to powdery mildew (PMR) at Delhi as well as in the Lahaul Valley of Himachal Pradesh in high Himalayas (elevation approximately 4000 m). The Lahaul Valley is known for its intense powdery mildew pressure, where even confirmed PMR genotypes can display dense fungal growth on the leaves and stipules (but never on the stem, peduncles, and pods). The Dharwad and Kanwa accessions give a similar reaction in Lahaul.
To confirm the nature of resistance and define allelism, the Dharwad accession was crossed with a powdery mildew susceptible (PMS) strain, Pusa 10, and the PMR variety P 1542. The major gene constellations and features of the three parents are as follows:
Dharwad: A, I, Le, R, PMR, small pod, small seed, opaque mottled testa
Pusa 10: a, I, le, R, Er, very large and thick pod, bold seed, translucent spotless testa
P 1542: a, i, le, r, er, long and slender pod, bold seed, translucent spotless testa
The observations recorded in the F1 and F2 generations are given below: _____Cross ___________Fj__________ ______________F2____________________
Pusa 10 x Dharwad A Le PMS mottled testa Segregation for all traits except I and R
P 1542 x Dharwad A Le PMR mottled testa Segregation for all traits except PMR
These observations confirm that the Dharwad accession and P 1542 carry a common recessive gene for powdery mildew resistance (absence of segregation). The PMS phenotype in the F1 and a 3:1 (susceptible:resistant) segregation in the F2 of the cross Pusa 10 x Dharwad also confirmed the monogenic recessive nature of the PMR trait in Dharwad.
The materials derived from the two crosses were advanced as single plant progenies (single seed descent), and are now at an F6 stage. Over the years, all possible gene combinations have been evolved. The PMR nature of the P 1542 x Dharwad derivatives has been confirmed repeatedly at Delhi and Lahaul. These results provide compelling evidence for a common PMR gene operating in Dharwad accession and the common P. sativum genotypes that are being used in breeding for powdery mildew resistance.
The results also suggests that only a single locus controlling powdery mildew resistance exists in the genus Pisum. This locus was discovered by Harland (1), recognized as Er1, and mapped on chromosome 6 (2). The putative gene Er2 has never been mapped.
1. Harland, S.C. 1948. Heredity 2: 263-269.
2. Sarala, K. 1993. Ph. D. Thesis. Indian Agric. Res. Inst., Delhi, India: 109+xi.