The pea rms2-1 rms4-1 double-mutant phenotype is transgressive
Murfet, I.C. and Symons, G.M. School of Plant Science, University of Tasmania
Hobart TAS 7001, Australia
Mutant recessive alleles at six Rms (ramosus) loci in pea confer increased branching, and we recently reported on the phenotype of several double-mutant combinations (1). For the cross K524 (rms2-1) x K164 (rms4-1) (both mutants ex cv. Torsdag), we found no evidence of transgression but with n = 24 there was a 20% chance that no double-mutant plant was actually present in this small F2 population. To obtain a definitive answer, we have now grown F3 progeny from two F2 plants with a single-mutant rms4 phenotype. These plants were identified by their ramosus phenotype but normal pod shape like cv. Torsdag. In contrast, pure rms2-1 plants have convexly-curved pods (1).
Both F3 progenies segregated some plants with curved pods and a tendency to wilt indicating they were homozygous for allele rms2-1. Two of these candidate double-mutant plants were subsequently confirmed as pure rms2-1 rms4-1 with greater than 99% confidence (n = 7) by backcrossing to K164 and K524. All backcross plants had a mutant phenotype.
The rms2-1 rms4-1 double-mutant plants were also easily recognizable by their profuse branching, and it is now clear that this double mutant has a transgressive phenotype. Based on either of the two branching indices ratio of total lateral length to main-stem length or number of laterals exceeding 1 cm in length, the double mutant showed a 2 to 3-fold increase in branching over either single mutant (Fig. 1).
Fig. 1. Two-way plot of branching indices ratio of total lateral length to main-stem length (TLL:TL) and number of laterals ³ 1 cm for initial line cv. Torsdag, single mutants K524 (rms2-1) and K164 (rms4-1), and F3 progeny from two cross K524 x K164 F2 plants with genotype Rms2/- rms4/rms4. Photoperiod 18 h.
1. Murfet, I.C. and Symons, G.M. 2000. Pisum Genetics 32: 33-38.