Expression of senescence in Neurospora intermedia

Kalilo cytoplasms of Neurospora intermedia have been shown to express senescence in two ways. First, by the previously reported way of death in a subculture series, and second, as reported here, by growth cessation in a 50-cm race tube. Only those cultures that are sufficiently far advanced in the development of senescence will stop growth in the length of a race tube. Resumption of growth occurs in most cases of growth arrest in race tubes. Although in subculture series growth resumption is rare, there is probably a similar basis: mitochondrial DNA (mtDNA) studies on one such case showed that growth resumption is associated with a resurgence of normal mtDNA and a decline of abnormal genomes. When senescent cultures in race tubes were sampled by removing mycelial cores, longer grown cultures were shown to be able to support less growth than younger cultures of the same genotype, and the growth front was generally able to support less growth than other regions. Therefore senescence in both transfer series and in race tubes involves the accumulation of genetic factors unable to suppport continuous growth. The expression of senescence is considered to be more efficient in subculture series than in race tubes. In such series, conidia or aerial hyphae work equally well as transfer inocula, but 1-mm hyphal tips cut from growing mycelium do not promote senescence when used as inocula. Furthermore, varying the transfer interval does not affect senescence. It is concluded that there is some feature of the development or germination of quiescent cells that enhances the expression of senescence in Neurospora.Key words: Neurospora, senescence, mitochondria, growth.