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Cultivated Alfalfa at the Diploid Level: Origin, Reproductive Stability, and Yield of Seed and Forage¹

Cultivated alfalfa (Medicago sativa L.) at the diploid level (CADL) was developed from cultivated tetraploids over a 10-year period using haploidy, with breeding and selection. The goal was to produce a diploid form for research which was seed-reproduced and stable in chromosome number. The 4x-2x cross method was used to obtain 2x haploids, which, although relatively sterile, could be used as females in initial crosses with 2x M. falcata F₁ hybrids thus obtained were fertile and were backcrossed as male parents to 2x haploids. CADL populations were selected for seed yield in between backcrosses. The proportion of cultivated germplasm increased by half with each backcross progressing from 50% in initial F₁ hybrids with M. falcata to 98.44% in the fifth backcross to haploids, which is the current level. Selection for fertility was very effective and resulted in cross- and self-fertility levels similar to tetraploid alfalfa in terms of seed number. Seven CADL populations and 2x M. falcata were screened for plants producing restitution (2n) gametes and for tetraploid plants in the diploid populations. Early populations contained a few plants producing 2n gametes and 2% tetraploids, while recent populations, which have had more selection for diploid fertility, contained no tetraploids. Hence, true breeding diploid CADL populations were obtained. A population with 87.5% cultivated germplasm was tested for forage yield and produced 72% as much as Vernal in three cuttings in the first 2 production years. Research applications of CADL were discussed.

Key Words: Chromosome • Gamete • Haploid • Polyploid • Medicago sativa L.

² Professor and graduate research assistant, Dep. of Agronomy, Univ. of Wisconsin, Madison.

Received for publication May 3, 1978.