Palaeoploidization and adaptation: An evolutionary strategy among pteridophytes with a reference to Ophioglossum L

摘要

Homosporous pteridophytes have extremely high chromosome numbers relative to flowering plants, but the species with the lowest recorded chromosome numbers show gene expression patterns of diploid organisms, suggesting that these probable ancient polyploids are now completely diploidized. Assuming palaeopolyploidy in the present-day diploids, their genomes have seemingly undergone wholesale diploidization over vast intervals of geological time This evolutionary strategy of high chromosome numbers and genetic diploidy has been briefly reviewed on two points: First, these plants are ancient polyploids that have undergone extensive gene silencing to produce ‘genetic diploids’, and second, they may have increased chromosome numbers also through chromosomal aberrations. For this accomplishment it is hypothesized that the allopoloyloidy (natural hybridization) must have played a vital role because this rare natural phenomenon exposes new genes and chromosomes for functional interaction through division cycles and meiotic screening. The incumbent individual segregate(s) struggle for survival by producing numerous morphological and chromosomal combinations as revealed by population cytogenetic studies carried out on two pteridophytic genera, a heterosporous lycophyte Isoetes and another a primitive homosporous plant also referred as a fern, Ophioglossum . In both these unrelated genera, hybrid segregates have exhibited uniform story of struggling variants and have supported the concept “that polyploidization, more effectively allopolyploidy can trigger abrupt changes at the genomic DNA levels, thereby causing variations in cytosine methylation and also influence heterochromatinization on the chromatin. Appearance of new chromosomes within the genome are being understood to have been evolved primarily on account of allopolyploidy and new individuals with variables are obviously facing diversified selection pressures. This is also conceived that the giant heterosporous lycopods of Carboniferous must have been polyploids that had dominated the homosporous pteridophytes during Carboniferous (300 mya) until they had to face biological extinction on account of drastic climatic turmoil during the Mesozoic. After that as estimated by palaeobotanists, giant occupancy on marshy land gradually culminated to greatest reduction series of evolution finally to herbaceous lycopods such as quillworts. Also, the homosporous ferns emerged as dominating vegetation (polyploidization might have been helpful) at the same time when angiosperms arose as contemporary flora. The reduction series imposed on heterosporous plants probably might have also involved allopolyploidy and other mechanisms resulting in downsizing of genomes. In many extant genera, as exemplified here on homosporous palaeopolyploid Ophioglossum L and heterosporous lycopod, Isoetes L allopolyploidy offers both superior as well as inferior genotypes with many variable at morphological and chromosomal levels so that reproductive fitness is decisively handled by natural selection. Autopolyploidy in nature rarely follows allopolyploidy so natural segregates often have irregular meiosis. In Summary, it is emphasized that polyploidy has been a fundamental mechanism “prima facie” responsible to trigger speciation processes.