Maize
Genetics Cooperation Newsletter 80. 2006.
In last year�s MNL (79:45), we reported that the mutable r1 haplotype-specific aleurone color enhancers Fcu and arv‑m594 map to the same location and are probably allelic. We obtained additional mapping data for this pair of factors, and tested additional factors for linkage. We tested the mutable factors Fcu with arv‑m594, arv‑m594 with arv‑m694, and Fcu with arv‑m694. We also tested the stable full color enhancer Arv‑V628#16038 with a full color Fcu revertant, Fcu‑R2003‑2653‑6. All tests were conducted as follows: Lines homozygous for the two factors, and homozygous for either r1‑g, or for a responsive r1 haplotype, were crossed together. The resulting F1�s were outcrossed to the responsive r1 haplotype R1‑r(Venezuela559‑PI302355) without any factors present. The parental classes and the double factor recombinant class would be expected to have mutable or full colored aleurones, and the recombinant class lacking both factors would be expected to have stable pale or colorless aleurones. Kernels from these crosses were scored for the presence of sectoring in the case of the mutable factors, or full color in the case of the stable factors, and exceptional stable pale or colorless kernels were planted last summer and the resulting plants self-pollinated and outcrossed to R1‑r(Venezuela559‑PI302355) in order to confirm the genotypes of these kernels. Since the only recombinant class that can be detected by these experiments is the class lacking both factors, we doubled the number of kernels in this class in order to account for the double mutant class for the purpose of calculating linkage values. The results are presented below:
Fcu with arv‑m594. If we combine the data presented last year with the data collected this year, we find no putative crossovers in a population of 3,223 kernels. Thus, we calculate that these two factors are separated by less than 0.06 centiMorgans (cM), and are likely allelic.
arv‑m594 with arv‑m694. We isolated four stable pale kernels from a population of 1,620 kernels in this test. However, upon further testing, only two of them proved to be truly stable (not carrying a mutable factor). Thus, we calculate an apparent map distance between these two factors of 0.25 � 0.12 cM. We could not rule out these kernels being the result of self-contamination by the R1‑r(Venezuela559‑PI302355) tester used as the female in the test crosses, but the plants grown from the exceptional kernels appeared to be vigorous outcross plants and not the result of self-contamination by the tester, which is in a W22 background. We also cannot rule out the possibility that the exceptional kernels represent stable derivatives of the mutable factors and not crossovers. Thus, we conclude that these two factors are either very tightly linked, or more likely, allelic.
Fcu with arv‑m694. We isolated eight stable pale or colorless kernels from a population of 2,372 kernels in this test. Upon further testing, seven of these proved not to be crossovers, and one did not survive to pollination and could not be tested. Thus we calculate that these two factors are separated by less than 0.08 cM, and are likely allelic.
All three mutable factors were tested for linkage with each other, and few potential crossovers were identified. Those that were identified could actually be stable null derivatives of the mutable factors. We conclude that Fcu, arv‑m594, and arv‑m694 map to virtually the same position, and are likely allelic, if not identical, to each other. For information on the origin of these factors, see Stinard, MNL 79:45.
Arv‑V628#16038 with Fcu‑R2003‑2653‑6. We isolated six stable pale or colorless kernels from a population of 1,887 kernels in this test. Upon further testing, only one of these proved to lack both enhancers. Thus, we calculate a map distance between these two factors of 0.11 � 0.07 cM. Again, we could not rule out self-contamination by the R1‑r(Venezuela559‑PI302355) tester used as the female in the test crosses, but the plant grown from the exceptional kernel appeared to be a vigorous outcross plant and not the result of self-contamination by the tester. This exceptional kernel could be a true crossover, although other possibilities such as mutation can�t be ruled out since these tests were not conducted using detectable flanking markers.
Arv‑V628#16038 is a naturally occurring enhancer of aleurone color
isolated from the r1 haplotype R1‑r(Venezuela638#16038) stock described by Van der Walt and Brink (Genetics 61:677-695, 1969). Fcu‑R2003‑2653‑6 is a revertant of Fcu isolated as described by Stinard (MNL
78:64-65). The potentially
identical map location of these two factors raises the interesting question of
the origin of the mutable and stable r1 aleurone color enhancers.
Did the mutable factors result from the insertion of a transposable
element in one of the naturally occurring stable factors, or did the stable
factors arise as a result of reversion or change of state of one of the mutable
factors? Only molecular analysis
will resolve this question. Since
we mapped Fcu (and thus the other
factors) to a particular segment on the long arm of chromosome 2 (see Stinard,
this MNL), we are in the process of trying to tag Arv‑V628#16038 with Ac
using one of Tom Brutnell�s mapped and characterized transposed Ac lines.
By this technique, we hope to eventually clone and characterize these
unique and interesting factors.
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