Maize
Genetics Cooperation Newsletter 80. 2006.
Members
of the Fcu and Arv class of r1 haplotype-specific aleurone color modifiers enhance aleurone color in
crosses to receptive r1
haplotypes such as r1‑cu
and R1‑r(Venezuela559‑PI302355)
(Stinard, MNL 77:77-79, 2003). So far, all responsive haplotypes
surveyed seem to be members of the class of r1 haplotypes that have seed color components containing
inverted repeats of coding sequences flanking rearranged Doppia transposable element sequences (Walker and Panavas,
Genetics 159:1201-1215, 2001), although not all haplotypes tested have been
subjected to molecular analysis.
Some
derivatives of R1‑st include
haplotypes that carry genes called Nc (Near
colorless) that consist of coding
sequences carrying Doppia sequences in
their promoter region (Matzke et al., Trends Plant Sci. 1:382-388, 1996). In order to test Fcu/Arv for ability to
enhance aleurone color expression of Nc
genes, a line homozygous for the stable enhancer Arv‑V628#16038 and the colorless aleurone r1 haplotype r1‑g(Stadler) was crossed to a line homozygous for r1‑sc:m6, a derivative of R1‑sc:134 that carries a single functional Nc gene (Eggleston et al., Genetics 141:347-360, 1995) and
has its main seed color component, Sc,
inactivated by a Ds transposable
element insertion (Jerry Kermicle, personal communication). Kernels from this cross had a lightly
mottled phenotype, whereas the r1‑sc:m6 parental line without enhancers had virtually colorless
aleurone. (Ac is not present in these lines.) Thus, Arv‑V628#16038 appears to enhance Nc expression. Kernels from
the F1 were planted this past summer, and the resulting plants self-pollinated. The resulting ears segregated for dark
and light mottled kernels, perhaps reflecting different dosages of the
enhancer, Arv‑V628#16038, and the
Nc receptor haplotype, r1‑sc:m6. Attempts
will be made to isolate lines homozygous for both enhancer and receptor.
Crosses
of Arv‑V628#16038 were also made
to the Nc line carrying the haplotype r1‑g(Nc)3‑5, which is a derivative of R1‑st that carries an Sc component inactivated by a
transposition-defective I‑R
element (Kermicle, Genetics 107:489-500, 1984), and likely carries three Nc genes, as it was derived directly from R1‑st, which also carries three Nc genes (Eggleston et al., 1995). All kernels from such crosses had virtually colorless
aleurone, but such a result is not unexpected since multiple copies of Nc genes in cis seem
to have an inhibitory effect on Nc
expression (Eggleston et al., 1995), and therefore any enhancing effect of Arv‑V628#16038 on Nc might go
undetected.
Factors
known to enhance Nc expression are
present in open-pollinated populations (Jerry Kermicle, personal
communication). A stock homozygous
for one such Nc enhancer isolated from
the land race Zapalote Chico, and homozygous for r1‑g(Stadler), was obtained from Jerry Kermicle. We outcrossed this stock to r1‑sc:m6, r1‑g(Nc)3‑5, and to R1‑r(Venezuela559‑PI302355). Crosses to r1‑sc:m6 produced a light mottled phenotype, and crosses to R1‑r(Venezuela559‑PI302355) produced full-colored kernels. On the other hand, crosses to r1‑g(Nc)3‑5 produced virtually colorless kernels. Thus, this Nc enhancer seems to behave similarly to Arv‑V628#16038. In order to
determine whether these factors might be closely linked or allelic, we crossed
the Nc enhancer to Arv‑V628#16038, and hope to have the results of mapping tests completed
by the end of next summer.
It remains an open question as to whether the Doppia sequences associated with coding sequences in responsive haplotypes have anything to do with the observed response to enhancers. It remains an attractive hypothesis, but only molecular analysis will resolve this question.
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