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Biomedical subjects

T F Sharbel

Publications and source records attributed to T F Sharbel.

8 recordsLinked to original sources
PubMed2006

Genetic diversity and reproductive biology in ecotypes of the facultative apomict Hypericum perforatum L.

Apomixis is a mode of asexual reproduction through seed. Progeny produced by apomixis are clonal replicas of a mother plant. The essential feature of apomixis is that embryo sacs and embryos are produced in ovules without meiotic reduction or egg cell fertilisation. Thus, apomixis fixes successful gene combinations and propagates high fitness genotypes across generations. A more profound knowledge of the mechanisms that regulate reproductive events in plants would contribute fundamentally to understanding the evolution and genetic control of apomixis. Molecular markers were used to determine levels of genetic variation within and relationship among ecotypes of the facultative apomict Hypericum perforatum L. (2n = 4x = 32). All ecotypes were polyclonal, being not dominated by a single genotype, and characterised by different levels of differentiation among multilocus genotypes. Flow cytometric analysis of seeds indicated that all ecotypes were facultatively apomictic, with varying degrees of apomixis and sexuality. Seeds set by haploid parthenogenesis and/or by fertilisation of aposporic egg cells were detected in most populations. The occurrence of both dihaploids and hexaploids indicates that apospory and parthenogenesis may be developmentally uncoupled and supports two distinct genetic factors controlling apospory and parthenogenesis in this species. Cyto-embryological analysis showed that meiotic and aposporic processes do initiate within the same ovule: the aposporic initial often appeared evident at the time of megaspore mother cell differentiation. Our observations suggest that the egg cell exists in an active metabolic state before pollination, and that its parthenogenetic activation leading to embryo formation may occur before fertilisation and endosperm initiation.

DNA Primers↗
PubMed2005

Biogeographic distribution of polyploidy and B chromosomes in the apomictic Boechera holboellii complex.

The Boechera holboellii complex comprises B. holboellii and B. drummondii, both of which can reproduce through sex or apomixis. Sexuality is associated with diploid individuals, whereas apomictic individuals are diploid or triploid and may additionally have B chromosomes. Using flow cytometry and karyotype analysis, we have shown that B chromosomes (a) occur in both diploid and triploid apomictic B. holboellii, (b) may occur in triploid B. drummondii, and (c) are dispensable for the plant. Both diploid and triploid karyotypes are found in multiple chloroplast haplotypes of both species, suggesting that triploid forms have originated multiple times during the evolution of this complex. B chromosome carriers are found in geographically and genetically distinct populations, but it is unknown whether the extra chromosomes are shared by common descent (single origin) or have originated via introgressive hybridization and repeated transitions from diploidy to triploidy. Diploid plants containing the Bs reproduce apomictically, suggesting that the supernumerary elements are associated with apomixis. Finally, our analyses of pollen size and viability suggest that irregular chromosome segregation in some triploid lineages may lead to the generation of diploid individuals which carry the B chromosomes.

Brassicaceae↗
PubMed2004

Is the aneuploid chromosome in an apomictic Boechera holboellii a genuine B chromosome?

The Boechera holboellii complex comprises B. holboellii and B. drummondii, both of which can reproduce through sex or apomixis. Sexuality is associated with diploidy, whereas apomictic individuals can either be diploid, aneuploid or triploid. Aneuploid individuals are found in geographically and genetically distinct populations and contain a single extra chromosome. It is unknown whether the supernumerary chromosomes are shared by common descent (single origin) or have originated via introgressive hybridizations associated with the repeated transition from diploidy to triploidy. Diploid plants containing the extra chromosome(s) reproduce apomictically, suggesting that the supernumerary elements are associated with apomixis. In this study we compared flow cytometry data, chromosome morphology, and DNA sequences of sexual diploid and apomictic aneuploids in order to establish whether the extra chromosome fits the classical concept of a B chromosome. Karyotype analyses revealed that the supernumerary chromosome in the metaphase complement is heterochromatic and often smaller than the A chromosomes, and differs in length between apomictic plants from different populations. DNA sequence analyses furthermore demonstrated elevated levels of non-synonymous substitutions in one of the alleles, likely that on the aneuploid chromosome. Although the extra chromosome in apomictic Boechera does not go through normal reductional meiosis, in which it may get eliminated or accumulated by a B-chromosome-specific process, its variable size and heterochromatic nature does meet the remaining criteria for a genuine B chromosome in other species. Its prevalence and conserved genetic composition nonetheless implies that this chromosome, if truly a B, may be atypical with respect to its influence on its carriers.

Aneuploidy↗
PubMed2001

Recurrent polyploid origins and chloroplast phylogeography in the Arabis holboellii complex (Brassicaceae).

Arabis holboellii is a North American member of the Brassicaceae that can reproduce via sex or apomixis. Previous studies have shown sexual individuals to be diploid, whilst apomictic individuals can be diploid (and aneuploid) or polyploid (typically 3x). Apomictic individuals can furthermore be facultative (i.e. both sexual and apomictic seed production in a single individual). Using flow cytometry, ploidy variation in 245 accessions of A. holboellii and A. drummondii from western North America and Greenland has been examined. Additionally, the chloroplast trnL intron region from each accession was sequenced for phylogenetic analysis of ploidy variation. Based upon 17 informative single nucleotide and insertion-deletion polymorphisms, we identified seven and 14 chloroplast haplotypes for A. drummondii and A. holboellii, respectively. Six of the haplotypes were found in both species. Ten of the chloroplast haplotypes were characterized by diploid, aneuploid, and triploid individuals, and thus we conclude that polyploidy has repeatedly and independently arisen within the species complex. As triploid individuals, which undergo normal meiosis, can only reproduce through apomixis, this may imply that the phenotype apomixis has also arisen multiple times. Arabis holboellii thus appears to have some predisposition to evolve apomictic reproduction.

Base Sequence↗
PubMed2000

B-chromosome evolution.

B chromosomes are extra chromosomes to the standard complement that occur in many organisms. They can originate in a number of ways including derivation from autosomes and sex chromosomes in intra- and interspecies crosses. Their subsequent molecular evolution resembles that of univalent sex chromosomes, which involves gene silencing, heterochromatinization and the accumulation of repetitive DNA and transposons. B-chromosome frequencies in populations result from a balance between their transmission rates and their effects on host fitness. Their long-term evolution is considered to be the outcome of selection on the host genome to eliminate B chromosomes or suppress their effects and on the B chromosome's ability to escape through the generation of new variants. Because B chromosomes interact with the standard chromosomes, they can play an important role in genome evolution and may be useful for studying molecular evolutionary processes.

Animals↗
PubMed2000

Genetic isolation by distance in Arabidopsis thaliana: biogeography and postglacial colonization of Europe.

Arabidopsis thaliana provides a useful model system for functional, evolutionary and ecological studies in plant biology. We have analysed natural genetic variation in A. thaliana in order to infer its biogeographical and historical distribution across Eurasia. We analysed 79 amplified fragment length polymorphism (AFLP) markers in 142 accessions from the species' native range, and found highly significant genetic isolation by distance among A. thaliana accessions from Eurasia and southern Europe. These spatial patterns of genetic variation suggest that A. thaliana colonized central and northern Europe from Asia and from Mediterranean Pleistocene refugia, a trend which has been identified in other species. Statistically significant levels of multilocus linkage disequilibrium suggest intermediate levels of disequilibrium among subsets of loci, and analysis of genetic relationships among accessions reveal a star or bush-like dendrogram with low bootstrap support. Taken together, it appears that there has been sufficient historical recombination in the A. thaliana genome such that accessions do not conform to a tree-like, bifurcating pattern of evolution - there is no 'ecotype phylogeny.' Nonetheless, significant isolation by distance provides a framework upon which studies of natural variation in A. thaliana may be designed and interpreted.

Arabidopsis↗
PubMed1998

B-chromosome origin in the endemic New Zealand frog Leiopelma hochstetteri through sex chromosome devolution.

The endemic New Zealand frog Leiopelma hochstetteri has variable numbers of mitotically stable B chromosomes. To assess whether the B chromosomes were derived from the autosome complement, they were isolated by micromanipulation and their DNA amplified by degenerate oligonucleotide primed PCR. Southern hybridizations of B chromosome DNA probes to genomic DNA from males and females characterized by differing numbers of B chromosomes demonstrated that the B chromosomes were derived from the univalent W sex chromosome characteristic of North Island populations. The presence of homologous B chromosome specific sequences from geographically distinct populations indicates a single origin of the B chromosomes. Furthermore, a primitive homology shared by B chromosomes and the W sex chromosome from an ancestral WZ/ZZ karyotype, which is still present in frogs from Great Barrier Island, shows that the B chromosomes originated soon after the univalent W sex chromosome had originated. Sequence analysis revealed that B chromosome DNA is composed of repeat sequences and has the potential to form stable hairpin structures. The molecular dynamics of these structures may reflect an inherent propensity to undergo rapid change in nucleotide sequence and chromosome structure.

Animals↗