MILAN, ITALY
University of Milan

Linkage data for sml

Manzotti P.S, Gavazzi G.

The sml gene is a recessive mutation affecting shoot apical meristem maintenance and lateral organ formation. Its introgression in different genetic backgrounds has highlighted the epistatic interaction between sml and the unlinked distorted growth (dgr) locus. Seeds homozygous for both sml and dgr loci have a shootless phenotype whereas Dgr/-sml/sml seeds produce plants with altered fillotaxis and abnormal leaf morphogenesis.

Previous data had shown that sml lies on the long arm of chromosome 10 and it was established a distance of 21 cM between sml and the molecular marker UMC1084 (Pilu et al.,2002 Plant Physiology 128, 502-511); in order to define if its position is centromeric proximal or distal, we made a three-point linkage test of sml using the chromosome 10 markers r and o7.

Since homozygous sml seedlings are lethal, the test was set up as a modified testcross, as outlined hear below.

Heterozygous sml R-st O7 / Sml r o7 females were outcrossed to Sml r O7/ Sml r O7 male parents and kernels from the cross were separated into stippled (R-st/r) and colorless (r/r) classes, planted, and the resulting plants self-pollinated. The F2 ears were scored for the presence of o7 and sml (upon germination of a sample of 50 seeds from each ear), thus allowing to trace back the chromosomal constitution of the outcrossed heterozygous females in terms of the three markers.

The results, obtained by the analysis of 145 ears, were tabulated, and linkage values were calculated (see Table 1).

Table 1: Modified three point linkage data for sml-r-o7

Region	phenotype	No. of ears	Totals
0	sml R-st O7	50
	Sml r o7	33	83
1	sml r o7	11
	Sml R-st O7	13	24
2	sml R-st o7	14
	Sml r O7	16	30
1+2	sml r O7	6
	Sml R-st o7	2	8
total			145

The gene order and distances so obtained are as follows:

sml – 22 + 3.4 - r – 26 + 3.6 - o7

A successive test was performed using the chromosome 10 markers r and v18.

Heterozygous Sml R-r V18 /sml r v18 plants were selfed and the kernels obtained divided into coloured and colourless, germinated and scored for the presence of sml and v18. The results obtained are shown in the table 2.

Table 2: Linkage data between sml, r and v18 as determined in the progeny of Sml R-r V18 /sml r v18 selfed plants

	phenotype
	Sml R	Sml r	sml R	sml r
n.	697	55	77	138

	R V18	R v18	r V18	r v18
n.	619	135	51	89

	Sml V18	Sml v18	sml V18	sml v18
n.	588	164	82	60

They allow to establish this linkage results:

sml – 16+ 1.3 – r – 24,5 + 1.7 – v18