Full paperSpore formation and karyological characterization of basidiosporogenesis, meiosis, post-meiotic and nuclear migration mitosis in Coprinus comatus
Introduction
Breeding of mushroom strains is usually carried out by a crossbreeding method using their heterothallic life cycle. Binuclear heterokaryon efficiently produces fruiting bodies and forms new basidiospores after meiosis. Using this mechanism, we have bred excellent dikaryons by crossing among useful monokaryons. Therefore, mushrooms that exhibit secondary homothallism, such as Agaricus bisporus (J.E. Lange) Imbach, form heterokaryons at the stage of basidiospores, and it is extremely difficult to perform crossbreeding as described above (Chakravarty, 2011). Therefore, it is very important for breeding to understand the life cycle in each mushroom species. The life cycle of the basidiomycete Coprinopsis cinerea (Schaeff.) Redhead, Vilgalys & Moncalvo is well known as a typical heterothallic mushroom. In this mushroom, the basidiospore is binuclear, but the monokaryon is produced from dikaryotic basidiospores (Kües, 2000).
Coprinus comatus (O.F. Müll.) Pers., the shaggy ink cap, is a common edible mushroom usually seen on lawns, in fields and on road sides. In China, people recognize it as a commercial mushroom. However, in Japan, C. comatus is not commonly consumed. Furthermore, like Co. cinerea, it is difficult to keep fresh. Consequently, C. comatus remains unpopular. Besides its edible value, C. comatus is regarded as a medicinal fungus, possessing antioxidant properties and hypoglycemic activity, and showing inhibition of adipocyte differentiation (Han, Yuan, Wang, & Li, 2006; Li, Lu, Suo, Nan, & Li, 2010; Park et al., 2014). However, despite this importance, its life cycle has not been reported.
In the past, researchers used Co. cinerea to study meiotic division. Coprinopsis atramentaria (Bull.) Redhead, Vilgalys & Moncalvo and C. comatus are relatively rare for use in research. However, in C. comatus, meiosis and nuclear migration are unclear, which has led to difficulties in breeding (Lu & Chiu, 1978; Lu & Raju, 1970; Raju & Lu, 1973). In this research, we focus on the karyological characterization of basidiosporogenesis and nuclear migration.
Section snippets
Strains and cultivation
Isolates (TUFC30838) were obtained from the Fungus/Mushroom Resource and Research Center, Faculty of Agriculture, Tottori University, Tottori, Japan. The wild-type dikaryotic strain TUFC30838 was cultivated on a rice straw substrate. The substrate formulation consisted of rice straw and wheat bran at a dry weight ratio of 8:2, followed by 100 g wet weight of 5% calcium carbonate to adjust pH. Moisture content of the substrate was adjusted to 65%. Into one 250-mL plastic bottle was placed 70 g
Results
Sterigmata formation, spore maturation, spore discharge, and color change were observed (Fig. 1). Observation revealed the occurrence of stages from karyogamy, meiosis, post-meiotic mitosis, and nuclear migration events to spore formation (Fig. 2). After differentiation into a basidium, two nuclei approach one another and fuse to a diploid nucleus. Following karyogamy, clear chromosome condensation can be observed (Fig. 2).
Spindle pole bodies were then stained and appear to be spherical at
Discussion
Karyological studies on mushrooms have mainly been performed on Agaricales, such as Lentinula edodes (Berk.) Pegler (Murakami & Takemaru, 1985) and A. bisporus (Kamzolkina et al., 2006). In L. edodes, post-meiotic mitosis occurs within the basidiospores. The daughter nuclei remain within the basidiospores, while their sister nuclei pass into the basidium. As a consequence, the mature basidium bears four uninucleate basidiospores and contains four nuclei (Murakami & Takemaru, 1985). However, in
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