Low temperature scanning electron microscopy of the ultrastructural development of zygospores and sporangiospores in Mycotypha africana, and the effects of cultural conditions on sexual versus asexual reproduction

Abstract

The ultrastructural details of sexual and asexual reproduction in Mycotypha africana were observed with low temperature SEM. Sexual reproduction was enhanced over asexual reproduction with a reduction of incubation temperature, the absence of light, and through the use of minimal amount of inoculation material. Repeated subculturing of actively growing zygosporic cultures resulted in a loss of zygosporic competence, which was reversible following 2–3 months storage of these cultures at 14–18 °C, 24 h dark. Sexual apparati were initiated by the contact of two zygophoric hyphae. Subapical contact resulted in formation of progametangia at the site of contact. Continued elongation of the zygophoric hyphae and additional, subsequent sexual contacts were observed. If sexual contact was apical, the apical region of the zygophoric hypha converted into a progamentangium, stopping elongation. The progamentangia enlarged, and gametangial septa were formed, isolating the gametangia, which fused, resulting in the formation of the zygosporangium. Zygospores developed internally and were observed externally by the rupture through the zygosporangium wall of the developing wart layer of the zygospore proper. Sporangiosporogenesis commenced with the formation of specialized aerial sporangiophores extending up to 5 000 μm. The apical regions of the sporangiophores enlarged, forming vesicles 300–400 μm long. Pedicels formed synchronously on the surface of the vesicles, the tips of which ballooned outward forming the sporangia. Two morphologically distinct types of sporangia were formed, spherical and elongate. These sporangia were formed in a highly regular spatial arrangement, alternating between the two sporangial types.