Comparative Histology of Cell Walls during Meiotic and Apomeiotic Megasporogenesis in Two Hexaploid Australasian Elymus Species
- John G. Carman ,
- Charles F. Crane and
- Oscar Riera-Lizarazu
Diplospory in Elymus rectisetus (Nees in Lehm.) Löve & Connor has been compared with megasporogenesis in the close sexual relative Elymus scabrus (R. Br.) Löve. Distinct differences in cell wall morphology were reported. The present study considered the cause of these cell wall differences by comparing cell wall behavior and composition during megasporogenesis in two E. rectisetus and two E. scabrus accessions (Australian wheatgrasses). Micropylar walls of meiotic megaspore mother cells (MMCs) during prophase and metaphase I (MI) were significantly thicker than those of apomeiotic MMCs in corresponding stages. Thickness of chalazal walls during these early stages did not differ. Micropylar, chalazal, and lateral walls of meiotic dyads and tetrads were significantly thicker than those of apomeiotic late stage MMCs, dyads, or binucleate monads. The walls of nucellar cells in E. rectisetus were generally thinner than those in E. scabrus, particularly during the latter stages of megasporogenesis. Callose, which normally envelops MMCs of angiosperms that form monosporic or bisporic female gametophytes, accumulated in and around cell walls of E. scabrus MMCs and was subsequently degraded as in other sexual species. Absence of callose in and around cell walls of MMCs of E. rectisetus appeared to be the major reason for the differences in MMC shape and cell wall thickness. In contrast, callose was detected at abnormal locations in both the nucellus and ovary wall of E. rectisetus. Meiosis with the normal accumulation of callose in and around the cell walls of the MMC and its dyad and tetrad derivatives was infrequently observed in E. rectisetus. Thus, callose accumulation appears to inherently accompany meiosis in these taxa.
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