Preliminary analysis of the pollen cell lineage

--R. Kelly Dawe and Michael Freeling

Rhoades observed that when purple anthers from a1-m plants were crossed to an a1 tester, about half transmitted the revertant phenotype through the gametes (Genetics 23:377-397). We have confirmed his result and have further shown that pollen derived from larger sectors--up to one half of the tassel -- transmit the revertant phenotype with the same likelihood as single anther sectors (50%, see Table 1). The results suggest that the cell lineages leading to the coloration of the anther wall and to the formation of the pollen mother cells are separable as early as the tassel two-cell stage.

Table 1. Transmission of Bz2 gametes from purple tassel sectors in bz2-m, Ac-2F11 plants1.
 
sector size # sectors crossed # heritable # not heritable
one half anther (purple half only) 7 3 4
whole anther 4 2 2
one half floret 12 6 6
whole floret 21 13 8
groups of florets (<half branch) 23 13 10
one half tassel branch 9 2 7
whole tassel branch 5 3 2
one eighth tassel 13 3 10
one fourth tassel 5 2 3
one half tassel 3 2 1
(subtotal) 102 49 53
       
more than one half tassel 3 3 0

1Seeds homozygous for bz2-m and heterozygous for Ac-2F11 and Pl were grown and screened for sectors. Pollen from locules or anthers of colored sectors was emptied onto the silks of homozygous bz2-stable, homozygous Ac testers. For sectors larger than one floret, three anthers from single florets were used. Only crosses yielding 30 or more seeds were tabulated. Sectors were considered heritable if 40% or more of the kernels from a cross were purple. Sectors were considered not heritable if less than 5% of the kernels were purple. Intermediate ratios (greater than 5% but less than 40% purple) were sometimes found, but are not considered here.

We have used the genetically unstable Ds insertion mutant bz2-m (M.G. Neuffer, MNL 27:67, 1953) to induce purple revertant sectors in the tassel. A number of Ac elements mapping to different places on chromosome 9S (in conjunction with Pl) were categorized according to when during tassel development excision events at bz2-m occurred. Most Acs permit frequent but late excision events only. However, the Ac present in the line that generated Adh1-2F11 (see Doring et al., MGG 193:199-204), Ac-2F11, is 20 map units from Wx and acts exceptionally early during plant development. In plants homozygous for bz2-m (and carrying Pl ), purple tassel sectors of one half tassel branch or larger occur in about 15% of the plants when Ac-2F11 is present.

Purple anthers on bz2-m plants generally transmit either 1) a low basal level of Bz2 pollen -- the reversion frequency of bz2-m pollen is about 5%, with roughly half of the reversion events occurring in bronze anthers -- or 2) 50% Bz2 pollen, the expected frequency from a germinal revertant heterozygote. The two possibilities occur with approximately equal frequency among sectors ranging in size from one half anther to one half tassel (Table 1; see "subtotal"). However, all of three sectors including more than half of the tassel were heritable. This result suggests that a single-celled tassel primordium forms developmentally distinct cell layers immediately following its first cell division. Additionally, results using B Pl stocks indicate that neither anther wall color nor the pollen mother cells are derived from the epidermal cell layer (L1).

When bz2-m plants were outcrossed to bz2 testers, the most common derivative kernels were bz2-stable, not purple. However, colorless sectors in bz2-m/bz2-stable (with Pl) tassels are extremely rare. Pollen shed from seven such colorless sectors of various sizes has been testcrossed and all seven transmitted the bz2-stable phenotype. Our working hypothesis is that anther wall color is derived from two different cell layers, one of which also generates the pollen mother cells. Under this hypothesis, colorless sectors are rare because bz2-stable events must occur simultaneously in two adjacent cell layers.


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