LABOULBENIALES - Agentschap voor Natuur en Bos

1.3.3. THE PERITHECI UMAscospores originate in the perithecium. TAVARES (1967, 1985) used peritheciumdevelopment and external structure of the perithecial wall to describe highertaxa. The perithecium is formed by divisions of one of the primary orsecondary receptacle cells. This gives at once the differentiation betweenprimary and secondary perithecium. In species without secondarily dividedreceptacle cells, the perithecium arises by divisions of cell II.We can distinguish the stalk and the perithecium:The stalk comprises two cells, the stalk cell (cell VI) and the secondary stalkcell (cell VII). The stalk cell is usually clearly visible; it subtends the basalcells of the perithecium (m, n and n’). Cell VII is directly derived from thestalk cell and is situated abaxially.The perithecium is more or less elongated and narrowed distally.Sometimes it is clearly differentiated into a rounded venter and a narrowneck (apex), terminating in an ostiole. The apex can rarely beasymmetric, so that the ostiole is situated subapical. The perithecial wallcells surrounding the ostiole often form distinct lips.The perithecium always consists of a well determined number of cells.Exceptions on this are genera in Herpomycetidae and Ceratomycetidae. Thelowest cells of the perithecium, just above the stalk cell and secondary stalkcell, are the basal cells. Usually, there are three such cells: the cell m is formedfrom cell VI, the cells n and n’ originate from cell VII. Divisions of cells n and n’result in three vertical rows of perithecial wall cells. The fourth row is formed bydivisions of cell m.The perithecial wall is double, i.e. composed of an interior and exterior part.The number and shape of the external wall cells plays an important role intaxonomy. The cells of the external wall are arranged in four vertical rows. Inprimitive genera, the number of cells in each of these rows is large; in othergroups, such as the Laboulbeniaceae, the vertical rows are composed of 4-5superposed cells. In contrast to the primitive genera, the cells in theLaboulbeniaceae are usually unequal in height, with the lower ones beingelongated and the apical ones being shortened.The basal part of the interior part of the perithecium contains the ascogonium.After fertilization through the trichogyne, it undergoes divisions resulting in oneto numerous ascogenous cells. The ascogenous cells give rise to new ascicontaining ascospores. The ascus wall is very thin; it disintegrates while still inthe perithecium during spore maturation.The trichogyne is an external thin appendage-like outgrowth of the youngperithecium. It is the place through which fertilization proceeds, a process inwhich different steps are recognized. At first, the trichophoric cell connectsthe trichogyne with the ascogonium within the perithecium. At an early stageof development, the perithecium is equipped with a trichogyne that receivesspermatia from the antheridia (TAVARES, 1985; DE KESEL, 1997). The next step inthe fertilization process is the fusion between the nuclei of the spermatium andthe ascogonium. The trichogyne deteriorates early after fertilization, except forthe basal part which often remains as a more or less prominent scar on thesurface of the mature perithecium.P a g e | 13