Kasbekar Durgadas P
Centre for DNA Fingerprinting and Diagnostics, Hyderabad 500 001, India.
J Genet. 2017 Jul;96(3):457-463. doi: 10.1007/s12041-017-0765-z.
When two lineages derived from a common ancestor become reproductively isolated (e.g. Neurospora crassa and N. tetrasperma), genes that have undergone mutation and adaptive evolution in one lineage can potentially become dysfunctional when transferred into the other, since other genes have undergone mutation and evolution in the second lineage, and the derived alleles were never 'tested' together before hybrid formation. Bateson (1909), Dobzhansky (1936), and Muller (1942) recognized that incompatibility between the derived alleles could potentially make the hybrid lethal, sterile, or display some other detriment. Alternatively, the detrimental effects seen in crosses with the hybrids may result from the silencing of ascus-development genes by meiotic silencing by unpaired DNA (MSUD). Aberrant transcripts from genes improperly paired in meiosis are processed into single-stranded MSUD-associated small interfering RNA (masiRNA), which is used to degrade complementary mRNA. Recently, backcrosses of N. crassa / N. tetrasperma hybrid translocation strains with wild-type N. tetrasperma were found to elicit novel ascus dysgenesis phenotypes. One was a transmission ratio distortion that apparently disfavoured the homokaryotic ascospores formed following alternate segregation. Another was the production of heterokaryotic ascospores in eight-spored asci. Lewis (1969) also had reported sighting rare eight-spored asci with heterokaryotic ascospores in interspecific crosses in Sordaria, a related genus. Ordinarily, in both Neurospora and Sordaria, the ascospores are partitioned at the eight-nucleus stage, and ascospores in eight-spored asci are initially uninucleate. Evidently, in hybrid crosses of the family Sordariaceae, ascospore partitioning can be delayed until after one or more mitoses following the postmeiotic mitosis.
当源自共同祖先的两个谱系变得生殖隔离时（例如粗糙脉孢菌和四孢脉孢菌），在一个谱系中发生突变和适应性进化的基因，当转移到另一个谱系中时可能会变得功能失调，因为其他基因在第二个谱系中发生了突变和进化，并且在杂种形成之前，衍生的等位基因从未一起“测试”过。贝特森（1909年）、多布赞斯基（1936年）和穆勒（1942年）认识到，衍生等位基因之间的不相容性可能会使杂种致死、不育或表现出其他一些有害特征。或者，在与杂种杂交中看到的有害影响可能是由于未配对DNA的减数分裂沉默（MSUD）导致子囊发育基因沉默所致。减数分裂中配对不当的基因产生的异常转录本被加工成与MSUD相关的单链小干扰RNA（masiRNA），用于降解互补mRNA。最近，发现粗糙脉孢菌/四孢脉孢菌杂种易位菌株与野生型四孢脉孢菌的回交引发了新的子囊发育异常表型。一种是传递率畸变，显然不利于交替分离后形成的同核体子囊孢子。另一种是在八孢子子囊中产生异核体子囊孢子。刘易斯（1969年）也报道过在相关属粪壳菌的种间杂交中罕见的带有异核体子囊孢子的八孢子子囊。通常，在脉孢菌属和粪壳菌属中，子囊孢子在八核阶段进行分配，八孢子子囊中的子囊孢子最初是单核的。显然，在粪壳菌科的杂交中，子囊孢子的分配可以延迟到减数分裂后有丝分裂后的一次或多次有丝分裂之后。
