Plant species diversity and polyploidy in islands of natural vegetation isolated in extensive cultivated lands

Natural polyploidy is often related to a longer life span, vegetative reproduction and higher competitive ability. In this paper, we test the possibility that these characteristics may favour the survival of polyploid taxa under conditions of long-term habitat fragmentation. In islands of natural vegetation isolated in extensive vineyards located in the South of France and in a large neighbouring area of natural vegetation, plant species richness and the relative abundance of polyploid taxa were assessed according to island size, isolation and vegetation structure. High species richness was observed, with numerous species restricted to the islands, suggesting that these may constitute refugia. However, species richness was not related to island size or to degree of isolation except for the flora of the woody areas. A very positive effect of area fragmentation on plant richness was observed, which is probably attributable to relatively low species overlap among the islands. Particularly high species richness was observed in open areas, provided that these were not extensively colonized by shrubs which seem to be responsible for local extinction of many annual taxa. Polyploids, which comprised mostly perennial herbs and woody species, were predominant in all the islands and in the large reference area. In open habitats invaded by shrubby species, a higher relative frequency of polyploids was observed in islands than in the reference area. Moreover, polyploid taxa were present in a larger number of islands than the diploid taxa, which were often restricted to a single island, suggesting that, after a long period of isolation, the polyploids may still have a lower probability of extinction. Evidence was obtained from vegetation structure analysis that diploid and polyploid annual herbs were restricted to open habitats and were both eliminated by shrubby species. Conversely, the diploid perennial herbs were also significantly affected by shrub colonization whereas the polyploids were mostly present in shrubby areas. This suggests that the higher competitive ability of polyploid perennial herbs may constitute a critical factor responsible for their wider distribution over the islands. We report the implications of our findings on conservation strategies, more particularly for a Mediterranean flora.     

Cytotaxonomy and evolution in Pennisetum section Brevivalvula (Gramineae) in tropical Africa

Chromosome numbers, meiotic pairing configurations, and fertility data are reported for 57 African collections representing five of the six taxa currently recognized in Pennisetum section Brevivalvula. All collections were polyploid from a basic number of x = 9 and exhibited high lrequencies of multivalents at metaphase I. Four of the five taxa include chromosomal races, either through aneuploidy or multiple polyploid levels. Data concerning pollen fertility and seed set suggest that agamospermy is present in these five taxa. Three taxa may be polymorphic for sexual reproduction. Integration of evidence concerning chromosomal and morphological variation in the section suggests possible hybrid origins for three of the six taxa.     

Segregation for sexual seed production in Paspalum as directed by male gametes of apomictic triploid plants

BACKGROUND AND AIMS: Gametophytic apomixis is regularly associated with polyploidy. It has been hypothesized that apomixis is not present in diploid plants because of a pleiotropic lethal effect associated with monoploid gametes. Rare apomictic triploid plants for Paspalum notatum and P. simplex, which usually have sexual diploid and apomictic tetraploid races, were acquired. These triploids normally produce male gametes through meiosis with a range of chromosome numbers from monoploid (n = 10) to diploid (n = 20). The patterns of apomixis transmission in Paspalum were investigated in relation to the ploidy levels of gametes. METHODS: Intraspecific crosses were made between sexual diploid, triploid and tetraploid plants as female parents and apomictic triploid plants as male parents. Apomictic progeny were identified by using molecular markers completely linked to apomixis and the analysis of mature embryo sacs. The chromosome number of the male gamete was inferred from chromosome counts of each progeny. KEY RESULTS: The chromosome numbers of the progeny indicated that the chromosome input of male gametes depended on the chromosome number of the female gamete. The apomictic trait was not transmitted through monoploid gametes, at least when the progeny was diploid. Diploid or near-diploid gametes transmitted apomixis at very low rates. CONCLUSIONS: Since male monoploid gametes usually failed to form polyploid progenies, for example triploids after 4x x 3x crosses, it was not possible to determine whether apomixis could segregate in polyploid progenies by means of monoploid gametes.     

Occurrence of polyploidy in the fishes

Polyploidy, the multiplication of entire setsof chromosomes beyond the normal set of two,has occurred extensively, independently, and isoften repeated in many groups of fish, from thesharks to the higher teleosts. While there areseveral ways that a polyploid fish can develop,environmental change and hybrid stabilizationmay play a large role in the initiation of anew polyploid species. Despite its prevalence,the importance of polyploidy in the evolutionof the fishes is unclear. Polyploidy is morecommon in the lower teleosts than the higherteleosts, possibly due to an advantage gainedthrough decreased specialization in the lowerteleosts, a decreased viability of polyploidyin the higher fish, or both. Polyploid fishcould gain an advantage over diploid fishthrough increased heterozygosity, divergence ofduplicate genes, and/or increased expression ofkey physiological proteins. While polyploidfish do not differ considerably from diploidsphenotypically, they may be at a disadvantage,or certain advantages may be lessened due to anoverall decrease in cell number. This papersummarizes all polyploid species of fish knownto-date, and discusses the possible roles andpathways for establishment of polyploidy in theevolution of the fishes.