Crosses of chromosomally normal female stocks by certain of the B-A translocations result in a fraction of the progeny with hypoploid endosperms that lack the BA paternal chromosome and that exhibit a small kernel phenotype. Lin (1982, Genetics 100:475) has shown that the small kernel effect does not merely result from the deficient state, since additional maternal doses will not restore the normal size.
Previous studies have shown that this phenotype did not function in a cell autonomous manner (Birchler, Genetical Research 36:111-116). The example studied involved the most extreme case of the small kernel phenotype, that is, TB-1La-5S8041 crossed to an a2 tester. Among the progeny were mosaic A2/a2 endosperms that had lost the paternal 1L-5S element as early as the first nuclear division. Even in these early events, there was no evidence of reduction in size in the a2 portion of the kernel. Determination of kernel mass did not reveal any reduction in these mosaics despite the fact that the small kernels, which were deficient for a paternal contribution at fertilization, were only 45% as large as the normal siblings.
In addition to the 1L-5S compound, 1L-3L translocations were noted to show nonautonomy in crosses to an a R-scm2 tester. Subsequent studies of 1L-5S crosses to an a2 R-scm2 stock confirmed the earlier observations on 1L-3L, that the mosaics were almost exclusively observed in kernels that carried only a single copy of the translocation in the endosperm. This fact was concluded from the observation that the mosaics always had anthocyanin development in the scutellum, and thus this class must have arisen from fertilization by pollen in which nondisjunction of the B centromere did not occur. Similar studies using a bz2 R-scm2 tester and TB-1La also indicated that the 1L small kernel effect showed nonautonomous behavior as evidenced by normal sized Bz2/bz2 mosaic kernels. These also occurred in endosperms resulting from inheritance of single dose of 1L from the TB-A parent.
While the above observations were consistent, they basically tested only the autonomy of the 1L small kernel effect regions. In the course of further studies on the effects of chromosomal dosage on gene expression, TB-10L18 and TB-10L19, both of which show the extreme 10L small kernel effect (Lin, 1982), were crossed to an r-r tester obtained from Jack Beckett. This female line appears to promote a high rate of nondisjunction or loss of the B-10L elements during endosperm development. Both translocation crosses produced numerous R/r endosperms. Thirty-three mosaic kernels were observed in crosses with TB-10L18. All had anthocyanin in the scutellum, indicating they arose from fertilizations by pollen carrying one dose of 10L in both sperm. All mosaics were within the normal sized range. The kernel mass means follow.
These observations indicate that the 10L as well as 1L small kernel effects exhibit nonautonomy.
James A. Birchler
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