Live cell confocal laser imaging studies on the nuclear behavior during meiosis and ascosporogenesis in Morchella importuna under artificial cultivation.

The commercial production of Morchella mushrooms, especially M. importuna, has been realized since 2012 in China, which facilitates the fundamental studies of Morchella spp. In this paper, the cytological characteristics at three stages of ascocarpic development and the nuclear behavior during meiosis and ascosporogenesis in cultivated strain M. importuna 1 was visualized by confocal laser scanning microscopy. The results suggested that the strain sporulated at sporulation stage of ascocarpic development. A total of six nuclear divisions typically took place during ascosporogenesis. The first and second divisions were meiotic in which the single diploid nucleus divided into four haploid nuclei. The subsequent mitosis gave rise to eight nuclei, and eight incipient ascospores with one nucleus in each spore were formed after spore delimitation in the clavate ascus. Then, the nucleus in most of the young ascospore underwent three successive mitoses producing 6-8 haploid nuclei in each mature spore, and thus the multinucleate ascospores in each ascus were all homokaryons. To the best of our knowledge, this is the first dynamic tracing study of nuclear behavior during meiosis and ascosporogenesis in cultivated morels, and the spore delimitation time is also the first report. The study will be beneficial for the genetics study and strain breeding of Morchella mushrooms.