Raju N B, Perkins D D
Department of Biological Sciences, Stanford University, Stanford, California 94305-5020, USA.
Fungal Genet Biol. 2000 Aug;30(3):213-21. doi: 10.1006/fgbi.2000.1217.
Immature asci of Coniochaeta tetraspora originally contain eight uninucleate ascospores. Two ascospore pairs in each ascus survive and mature, and two die and degenerate. Arrangement of the two ascospore types in individual linear asci is what would be expected if death is controlled by a chromosomal gene segregating at the second meiotic division in about 50% of asci. Cultures originating from single homokaryotic ascospores or from single uninucleate conidia are self-fertile, again producing eight-spored asci in which four spores disintegrate, generation after generation. These observations indicate that differentiation of two nuclear types occurs de novo in each sexual generation, that it involves alteration of a specific chromosome locus, and that the change occurs early in the sexual phase. One, and only one, of the two haploid nuclei entering each functional zygote must carry the altered element, which is segregated into two of the four meiotic products and is eliminated when ascospores that contain it disintegrate. Fusion of nuclei cannot be random-a recognition mechanism must exist. More study will be needed to determine whether the change that is responsible for ascospore death is genetic or epigenetic, whether it occurs just before the formation of each ascus or originates only once in the ascogonium prior to proliferation of ascogenous hyphae, and whether it reflects developmentally triggered alteration at a locus other than mating type or the activation of a silent mating-type gene that has pleiotropic effects. Similar considerations apply to species such as Sclerotinia trifoliorum and Chromocrea spinulosa, in which all ascospores survive but half the spores in each ascus are small and self-sterile. Unlike C. tetraspora, another four-spored species, Coniochaetidium savoryi, is pseudohomothallic, with ascus development resembling that of Podospora anserina.
四孢锥毛壳的未成熟子囊最初含有八个单核子囊孢子。每个子囊中的两对子囊孢子存活并成熟，另外两对死亡并退化。如果死亡是由一个在大约50%的子囊中第二次减数分裂时分离的染色体基因控制的，那么在单个线性子囊中两种子囊孢子类型的排列就是预期的那样。源自单个同核体子囊孢子或单个单核分生孢子的培养物是自育的，一代又一代地再次产生八孢子子囊，其中四个孢子解体。这些观察结果表明，两种核类型的分化在每个有性世代中从头发生，它涉及特定染色体位点的改变，并且这种变化发生在有性阶段的早期。进入每个功能合子的两个单倍体核中，只有一个必须携带改变的元件，该元件被分离到四个减数分裂产物中的两个中，并在包含它的子囊孢子解体时被消除。核融合不可能是随机的——必须存在一种识别机制。需要更多的研究来确定导致子囊孢子死亡的变化是遗传的还是表观遗传的，它是在每个子囊形成之前发生还是仅在产囊丝增殖之前在产囊体中发生一次，以及它是否反映了除交配型之外的位点上发育触发的改变或具有多效性作用的沉默交配型基因的激活。类似的考虑也适用于诸如三叶草核盘菌和刺状色层菌等物种，在这些物种中，所有子囊孢子都存活，但每个子囊中一半的孢子很小且自交不育。与四孢锥毛壳不同，另一个四孢子物种，萨氏锥毛壳菌是假同宗配合的，其子囊发育类似于鹅毛孢菌。
