A substantial part of the genome is expressed in the male haploid phase and the gametophytic-sporophytic genetic overlap has been estimated to exceed 60% (M. Sari-Gorla et al., TAG 72:42-47, 1986; P. Willing & J.P. Mascarenhas, Plant Phys. 75:865-868, 1984). However, all the studies have considered only mature pollen as male gametophyte and there is very little data concerning gene expression during microspore development.
In this report seven genes coding for enzymes were analyzed throughout pollen development, from 5 days after meiosis (DAM) to full maturity (anthesis). Six of these enzymes are present in mature pollen. Four (ADH-1, GOT-1, GOT-2, CAT-1) are multimeric and therefore suitable for the detection of the type of expression, haploid or diploid, of their encoding genes (Sari-Gorla et al., 1986). Two (GOT-3 and b-GLU-2) are invariant and possibly monomeric. For these only the onset of enzymatic activity during development was determined. One (b-GLU-1), absent in mature pollen, was included to verify its sporophyte-specificity
The analysis indicates that: ADH-1 and GOT-2 show haplo-diploid expression from the earliest stages examined. CAT-1 shows haplo-diploid expression only starting 7 DAM. No activity at all could be detected earlier. An additional anodal band showing catalase activity appears in immature microspores only. The electrophoretic mobility and the absence of heterodimers with CAT-1 suggest this isozyme may be CAT-4. For GOT-1, 3 bands typical of sporophytic genetic control appear in the early stages (up to 9 DAM), while the 2-band pattern expected for haploid expression was found from 9 DAM on. GOT-2 and b-GLU-2 were clearly detectable from 5 DAM. b-GLU-1 was absent throughout pollen development, thus confirming its sporophyte-specificity
On the whole the data indicate that:
- the male gametophyte is genetically largely independent from the sporophyte even in the first stages of development.
- in some cases (GOT-1) a sporophytic control over gametophytic functions may persist at least for the early steps of microspore development.
- some genes appear to be developmentally regulated (CAT-1, CAT-4) in the "pollen formation" phase. This last observation is confirmed by data regarding heat-shock response during microspore development.
Further analyses for a better characterization of the very early stages (0-5 DAM) are in progress.
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