I grew out progenies of AB-interchanges that had been crossed with the
3 unmapped genetic male steriles described by West and Albertsen (MNL 59:87)
to determine chromosome arm location of each male-sterile locus. These
progenies were from crossing either homozygous or heterozygous AB-interchanges
as male onto known heterozygotes of each of the 3 genetic male steriles.
I had decided to use known heterozygotes as female parents rather than
homozygous recessives because of the number of crosses that had to be made
with the AB-interchanges. Known heterozygotes for each of the male steriles
were obtained by selecting male-sterile plants from the stocks originally
used to describe ms22, ms23, and ms24 and intercrossing
with normal inbred B73. Given equal transmission of the Ms and ms
alleles through the female, the critical mapping cross will result in half
the hypoploids being fertile and half being sterile. Not all crosses with
every AB-interchange were possible, and no male-sterile hypoploids were
observed from crosses with ms22 and ms24. With ms23,
however, crosses with AB-interchanges involving the long arm of chromosome
3 yielded progenies in which the hypoploids segregated 1:1 for male sterility.
One was TB-3La; the other was compound interchange TB-3La-2S6270. Data
for these interchanges are shown below.
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The breakpoint for TB-3La is at 0.1 of the length of the long arm of chromosome 3. The other AB-interchange involving 3L is a compound interchange. These two interchanges provide us an approximation of the location of the ms23 locus on 3L. The distal breakpoint for TB-3La-2S6270 is at 0.6, and the proximal breakpoint is at 0.1. Therefore, the ms23 locus must be located within this chromosome segment. Crosses will be made this summer to specifically locate the ms23 locus. We will continue efforts to locate ms22 and ms24.
M.C. Albertsen
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