Maize Genetics Cooperation Newsletter vol 81 2007

Linkage data for sml

--Manzotti, PS; 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 phyllotaxis and abnormal leaf morphogenesis.

Previous data had shown that sml lies on the long arm of chromosome 10, and it was established that there is a distance of 21 cM between sml and the molecular marker umc1084 (Pilu et al., Plant Physiol. 128:502-511, 2002); 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 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 us to trace 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 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 were divided into coloured and colourless, germinated and scored for the presence of sml and v18. The results obtained are shown in 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

This allows us to establish this linkage result:

sml – 16 � 1.3 – r – 24.5 � 1.7 – v18

Please Note: Notes submitted to the Maize Genetics Cooperation Newsletter may be cited only with consent of authors.