Early flowering mutants Wt11790 and Wt11791 result from mutation at the Lf locus

Murfet, I.C.           Dept. of Plant Science, Univ. of Tasmania, Hobart Tas. 7001, Australia

The early flowering mutant lines Wt11790 and Wt11791 were selected by Dr W.K. Swiecicki (Plant Breeding Station Wiatrowo) following irradiation of cultivar Porta (Wt3519) with 200 r Nf. I report here the results of tests which show that early flowering in both these lines results from mutation of Lf to lf.

Materials and methods

Tests were conducted in the phytotron at Hobart. Seed of Porta and the mutant lines was kindly provided by Dr W.K. Swiecicki. The genotypes of the Wiatrowo lines were determined by crossing with Hobart lines of known flowering genotype, namely line 2 (late photoperiodic, Lf E Sn Dne hr) and line 60 (early photoperiodic, lf E Sn Dne hr). Plants were grown in 14 cm slim line pots in a 1:1 mixture of vermiculite:dolerite chips topped with 3 cm of sterilised peat/sand potting mixture. Nutrient (Hoaglands #1) was supplied once a week. Lateral shoots were regularly excised. Node counts commenced from the first scale leaf as node 1. Node of flower initiation was taken as the first node on the main stem to bear a flower initial regardless of whether or not the bud developed into a mature flower. The F₁ progenies in Table 1 with parental control lines were sown 12 Mar 1990 and the F₂ progenies with parental controls were sown 10 Aug 1990. All plants received 8 h of daylight at 20-25°C and 16 h of dark in night compartments at 16°C. Data for the controls were very similar from the two plantings and the results are combined in Table 1.

Results and discussion

Preliminary tests showed that Porta and the mutants have phenotypes L and EI, respectively, as defined by Murfet (1), i.e. Porta is a late photoperiodic type showing a quantitative increase (6-8 nodes) in the node of flower initiation in short day conditions while the two mutants are typical early photoperiodic types in which node of flower initiation is unaffected by photoperiod but days to first open flower is substantially delayed in short days. The characteristics of lines belonging to these phenotypic classes are illustrated in more detail elsewhere in this issue (4).

The phenotypic classification of Porta and its derivatives, and the fact that Wt11790 and Wt11791 both flower at nodes 11 and 12, indicates two possibilities. Porta may have genotype Lf E Sn Dne hr with the mutants arising from forward mutation of Lf to lf, or alternatively, Porta may be lf e Sn Dne hr with the mutants arising from back mutation of e to E. The latter possibility is excluded by the fact that F₁ of the cross Porta (Wt3519) x L60 (lf E Sn Dne hr) was late flowering (flowering node 18-22, Table 1) which indicates Porta carries gene Lf. (Lf is epistatic to E; 2). The first hypothesis was substantiated by all remaining data. The crosses of Wt11790 and Wt11791 with L60 bred true in the F_l and the F₂; all offspring flowered within the range of the parents (Table 1). The F₁ of cross Wt11791 (proposed genotype lf E Sn Dne hr) x L2 (Lf E Sn Dne hr) was late, flowering at nodes 24 and 25, while the F₂ segregated into 29 late plants flowering at nodes 19-28, 17 clearly early plants flowering at nodes 11-12 and 2 intermediate plants flowering at node 14. F₃ data (n=12 for each progeny) showed conclusively that thesetwo intermediate plants were Lflf heterozygotes because the majority of their offspring were phenotypically late. Thus the observed F₂ segregation of 31 Lf- and 17 lflf segregates is in agreement with the expected 3:1 ratio (Table 2, c² = 2.78, P > 0.05). The Lf locus is on chromosome 1 and shows fairly strong linkage with the basic gene for anthocyanin production, A (recombination fraction about 10%; 2, 3, 5). Cross Wt11791 (a lf) x L2 (A Lf) is in the coupling phase and the close linkage between the two loci (Table 2) is consistent with previous results and supports the conclusion that the late/early flowering difference in this cross is attributable to segregation at the Lf locus.

Table 1. Distribution of node of flower initiation for Porta (Wt3519), early flowering mutants Wt11790 and Wt11791, Hobart lines 2 (Lf E Sn Dne hr) and 60 (lf E Sn Dne hr), and F₁ and F₂ progenies for crosses between the Hobart and Wiatrowo lines. In crosses 11790 x 2 and 11791 x 2 F₂ plants flowering at nodes 13-16 were genotyped by growing F₃. Photoperiod 8 h.

Table 2. Dihybrid segregation data from the F₂ of crosses between Hobart line 2 (A Lf) and Wiatrowo lines 11790 (a lf) and 11791 (a lf).

***P< 0.001

As expected, cross WT11790 x L2 gave similar basic results to cross Wt11791 x L2 (Tables 1 and 2) except that there was an increased frequency of heterozygous Lflf plants flowering in the intermediate zone of nodes 13-16. This was already apparent in the Wt11790 x L2 F₁ where 3 of the 6 plants flowered at nodes 15-16 and 3 flowered at nodes 24-25 (Table 1). Eleven F₂ plants flowered in the intermediate zone and F₃ data again confirmed that 10 of these were Lflf heterozygotes. The joint segregation data again support the conclusion that the late/early flowering difference is attributable to segregation at the Lf locus because strong linkage is evident between the flowering gene concerned and marker A (Table 2).

The F₂ data from crosses L60 (A lf) x Wt11790 (a If) and L60 x Wt11791 (a lf) indicate that the mutant lf alleles in the two Wiatrowo lines are of very similar strength to the naturally occurring lf allele in the reference early photoperiodic line, L60. In cross 60 x 11790 the mean flowering node for A- segregates was 10.94 ± 0.09 (n=19) and 11.00 ± 0 (n=5) for the aa segregates. In cross 60 x 11791 the comparable data were 11.19 ± 0.14 (n=16) and 10.75 ± 0.16 (n=8). The slightly earlier flowering of the aa segregates in the latter cross may suggest lf¹¹⁷⁹¹ is a slightly earlier allele but that suggestion is not supported by the data for crosses with line 2 where it is cross 2x11790, not 2x11791 which contains the highest frequency of Lflf heterozygotes flowering in the intermediate zone. The tendency for heterozygotes for Lf alleles to flower intermediate between the pure forms is on record (2, 3).

Acknowledgements. I thank Dr W.K. Swiecicki for provision of the seed, Miss Heidi Dungey for technical assistance and the Australian Research Council for financial support.

1. Murfet, I.C. 1971. Heredity 26:243-257.

2. Murfet, I.C. 1971. Heredity 27:93-110.

3. Murfet, I.C. 1975. Heredity 35:85-98.

4. Murfet, I.C. and Ezhova, T.A. 1991. Pisum Genetics 23:19-25.

5. White, O.E. 1917. Proc. Amer. Phil. Soc. 56:487-589.