Raju N B, Perkins D D
Department of Biological Sciences, Stanford University, California 94305-5020.
Dev Genet. 1994;15(1):104-18. doi: 10.1002/dvg.1020150111.
Meiosis and ascospore development in the four-spored pseudohomothallic ascomycetes Neurospora tetrasperma, Gelasinospora tetrasperma, Podospora anserina, and P. tetraspora have been reexamined, highlighting differences that reflect independent origins of the four-spored condition in the different genera. In these species, as in the heterothallic eight-spored N. crassa, fusion of haploid nuclei is followed directly by meiosis and a postmeiotic mitosis. These divisions take place within a single unpartitioned giant cell, the ascus, which attains a length of > 0.1 mm before nuclei are enclosed by ascospore walls. Two basically different modes underlie the delivery of opposite mating type nuclei into each of the four ascospores in the different genera. In N. tetrasperma on the one hand, the mating type locus is closely centromere-linked. Mating types therefore segregate at the first meiotic division. The second-division spindles of N. tetrasperma overlap and are usually parallel to one another, in contrast to the their tandem arrangement in N. crassa. As a result, nonsister nuclei of opposite mating type are placed close together in each half-ascus and a pair is enclosed in each ascospore. In the Podospora and Gelasinospora species on the other hand, the second-division spindles are in tandem, with sister nuclei of opposite mating type associated as a pair in each half-ascus. It is established for P. anserina and inferred for P. tetraspora and G. tetrasperma that a single reciprocal crossing over almost always occurs in the mating type-centromere interval, ensuring that mating types segregate at the second meiotic division and that nuclei of opposite mating type are enclosed in each ascospore. Other differences are also seen that are less fundamental. Neurospora tetrasperma differs from the other species in the orientation of chromosomes and spindle pole body plaques at interphase II. Third-division spindles are oriented parallel to the ascus wall in Gelasinospora but across the ascus in Podospora and Neurospora. The two Podospora species differ from one another in nuclear behavior following mitosis in the young ascospores. In P. tetraspora, two of the four nuclei migrate into the tail cell, which degenerates, leaving one functional nucleus of each mating type. In P. anserina, by contrast, only one of the four nuclei moves into the tail cell, leaving the germinating ascospore with two functional nuclei of one mating type and one of the other. The pseudohomothallic condition with its heterokaryotic vegetative phase has significant consequences for both the individual organism and the breeding system. Genetic controls of development and recombination are complex.(ABSTRACT TRUNCATED AT 400 WORDS)
对四孢拟同宗配合子囊菌中的粗糙脉孢菌、四孢胶孢壳、灰盖鬼伞和四孢柄锈菌的减数分裂和子囊孢子发育进行了重新研究，突出了不同属中四孢条件独立起源所反映的差异。在这些物种中，如同异宗配合的八孢粗糙脉孢菌一样，单倍体核融合后紧接着进行减数分裂和减数分裂后有丝分裂。这些分裂发生在单个未分隔的巨型细胞即子囊内，在细胞核被子囊孢子壁包围之前，子囊长度超过0.1毫米。不同属中，将相对交配型细胞核传递到四个子囊孢子中的每一个有两种基本不同的模式。一方面，在四孢粗糙脉孢菌中，交配型基因座与着丝粒紧密连锁。因此，交配型在第一次减数分裂时分离。四孢粗糙脉孢菌的第二次分裂纺锤体重叠且通常相互平行，这与粗糙脉孢菌中它们的串联排列形成对比。结果，相对交配型的非姐妹核在每个半子囊内紧密排列在一起，并且每对子囊孢子中包含一对。另一方面，在柄锈菌属和胶孢壳属物种中，第二次分裂纺锤体是串联的，相对交配型的姐妹核在每个半子囊内成对相连。已确定在灰盖鬼伞中如此，推测在四孢柄锈菌和四孢胶孢壳中也是如此，即在交配型 - 着丝粒区间几乎总是发生一次单交换，确保交配型在第二次减数分裂时分离，并且相对交配型的核被包含在每个子囊孢子中。还观察到其他不太重要的差异。四孢粗糙脉孢菌在减数分裂II间期的染色体和纺锤极体斑的方向上与其他物种不同。在四孢胶孢壳中，第三次分裂纺锤体与子囊壁平行排列，但在柄锈菌属和粗糙脉孢菌中则横穿子囊。这两种柄锈菌属物种在年轻子囊孢子有丝分裂后的核行为上彼此不同。在四孢柄锈菌中，四个核中的两个迁移到尾细胞中，尾细胞退化，留下每种交配型的一个功能核。相比之下，在灰盖鬼伞中，四个核中只有一个迁移到尾细胞中，使得萌发的子囊孢子带有一种交配型的两个功能核和另一种交配型的一个功能核。具有异核营养阶段的拟同宗配合条件对个体生物体和繁殖系统都有重大影响。发育和重组的遗传控制很复杂。（摘要截断于400字）
