Biomedical subjects

Mark E Welch

Publications and source records attributed to Mark E Welch.

3 recordsLinked to original sources

PubMed2006

Variable populations within variable populations: quantifying mitochondrial heteroplasmy in natural populations of the gynodioecious plant Silene vulgaris.

Populations of mitochondria reside within individuals. Among angiosperms, these populations are rarely considered as genetically variable entities and typically are not found to be heteroplasmic in nature, leading to the widespread assumption that plant mitochondrial populations are homoplasmic. However, empirical studies of mitochondrial variation in angiosperms are relatively uncommon due to a paucity of sequence variation. Recent greenhouse studies of Silene vulgaris suggested that heteroplasmy might occur in this species at a level that it is biologically relevant. Here, we use established qualitative methods and a novel quantitative PCR method to study the intraindividual population genetics of mitochondria across two generations in natural populations of S. vulgaris. We show incidences of heteroplasmy for mitochondrial atpA and patterns of inheritance that are suggestive of more widespread heteroplasmy at both atpA and cox1. Further, our results demonstrate that quantitative levels of mitochondrial variation within individuals are high, constituting 26% of the total in one population. These findings are most consistent with a biparental model of mitochondrial inheritance. However, selection within individuals may be instrumental in the maintenance of variation because S. vulgaris is gynodioecious. Male sterility is, in part, regulated by the mitochondrial genome, and strong selection pressures appear to influence the frequency of females in these populations.

Genetic Variation↗

PubMed2003

The origin of ecological divergence in Helianthus paradoxus (Asteraceae): selection on transgressive characters in a novel hybrid habitat.

Diploid hybrid speciation in plants is often accompanied by rapid ecological divergence between incipient neospecies and their parental taxa. One plausible means by which novel adaptation in hybrid lineages may arise is transgressive segregation, that is, the generation of extreme phenotypes that exceed those of the parental lines. Early generation (BC2) hybrids between two wild, annual sunflowers, Helianthus annuus and Helianthus petiolaris, were used to study directional selection on transgressive characters associated with the origin of Helianthus paradoxus, a diploid hybrid species adapted to extremely saline marshes. The BC2 plants descended from a single F1 hybrid backcrossed toward H. petiolaris. The strength of selection on candidate adaptive traits in the interspecific BC2 was measured in natural H. paradoxus salt marsh habitat. Positive directional selection was detected for leaf succulence and Ca uptake, two traits that are known to be important in salt stress response in plants. Strong negative directional selection operated on uptake of Na and correlated elements. A significant decrease in trait correlations over time was observed in the BC2 population for Na and Ca content, suggesting an adaptive role for increased Ca uptake coupled with increased net exclusion of Na from leaves. Patterns of directional selection in BC2 hybrids were concordant with character expression in the natural hybrid species, H. paradoxus, transplanted into the wild. Moreover, the necessary variation for generating the H. paradoxus phenotype existed only in the BC2 population, but not in samples of the two parental species, H. annuus and H. petiolaris. These results are consistent with the hypothesis that transgressive segregation of elemental uptake and leaf succulence contributed to the origin of salt adaptation in the diploid hybrid species H. paradoxus.

Adaptation, Biological↗

PubMed2002

Patterns of genetic variation suggest a single, ancient origin for the diploid hybrid species Helianthus paradoxus.

Experimental and comparative evidence implies that homoploid hybrid speciation is a reproducible process, mediated in part by ecological selection. Here, molecular data from the chloroplast genome and 17 nuclear microsatellite loci were employed to determine whether a well-documented homoploid hybrid species, Helianthus paradoxus, has arisen multiple times. Helianthus paradoxus is ecologically divergent from its parental species, and has a disjunct geographic distribution consistent with multiple origins. The molecular data, however, strongly support a single hybrid origin. First, all sampled populations of H. paradoxus are fixed for a single chloroplast DNA (cpDNA) haplotype, whereas local populations of both parental species, H. annuus and H. petiolaris, have multiple cpDNA haplotypes. Second, H. paradoxus populations form a single, well-supported clade (99.8% bootstrap support) in a neighbor-joining tree based on microsatellite allele frequencies. The microsatellite data also tentatively place the origin of H. paradoxus between 75,000 years and 208,000 years before present, indicating that anthropogenic disturbance likely did not play a role in the formation of this species. Finally, the genetic structure of this species is not consistent with passive riparian dispersal, which has been suggested for other wetland plant species, but may be explained by dispersal mechanisms implicated for H. annuus, such as large migratory mammals.

Alleles↗