Chloroplast capture, the introgression of a chloroplast from one species into another, has been frequently suggested as the explanation for inconsistencies between gene trees based on nuclear and cytoplasmic markers in plants. We use a genetic model to determine the conditions for capture to occur, and we find that they are somewhat more general than those given in earlier verbal arguments. Chloroplast capture can occur if cytoplasm substitution provides an advantage in seed production. This can happen through reallocation to the female function when cytonuclear incompatibilities cause partial male sterility, but also under more general conditions. Capture is promoted by nuclear incompatibilities between the two genomes (or a low heterosis in F1 hybrids) and by partial selfing when hybridization causes a decrease in the selfing rate and inbreeding depression is strong. We discuss empirical predictions that can be used to test this mechanism.

A model for chloroplast capture

The genus Boechera is a widespread North American group with great potential for studies of ecology and evolution: Boechera is closely related to Arabidopsis and exhibits different ecological and reproductive strategies. Boechera stricta (previously Arabis drummondii) is a morphologically and genetically well-defined, perennial crucifer species. Fifteen natural populations of diploid individuals from the Rocky Mountains were analysed using 21 microsatellite loci. In accordance with our expectation for this predominately inbreeding species, a high F IS value (0.89) was observed. Furthermore, populations of B. stricta were highly differentiated, as indicated by F ST = 0.56. Three clusters were identified using structure- the majority of populations belonged to either the Northern or Southern cluster. Together, the north-south partitioning and evenness of genetic variation across the two clusters suggested multiple refugia for this perennial herb in the Rocky Mountains. Pleistocene glaciation, together with the topographically and climatologically heterogeneous cordillera, has profoundly influenced the genetic architecture of B. stricta. Genetic population structure was also influenced by relatively recent genome admixture at two levels: within species (involving individuals from the Northern and Southern clusters) and between species (with the hybridization of B. stricta and Boechera holboellii). This complexity of population structure at presumably neutral microsatellite loci located throughout the genome in B. stricta provides a baseline against which to test whether functional genetic variation is undergoing local adaptive evolution throughout the natural species range.

Geographic patterns of microsatellite variation in Boechera stricta, a close relative of Arabidopsis.

High-latitude ecotonal populations at the species margins may exhibit altered patterns of genetic diversity, resulting from more or less recent founder events and from bottleneck effects in response to climate oscillations. Patterns of genetic diversity were investigated in nine populations of the conifer black spruce ( Picea mariana [Mill.] BSP.) in northwestern Quebec, Canada, using seed-dispersed mitochondrial (mt) DNA and nuclear (nc) DNA. mtDNA diversity (mitotypes) was assessed at three loci, and ncDNA diversity was estimated for nine expressed sequence tag polymorphism (ESTP) loci. Sampling included populations from the boreal forest and the southern and northern subzones of the subarctic forest-tundra, a fire-born ecotone. For ncDNA, populations from all three vegetation zones were highly diverse with little population differentiation ( θ N = 0.014); even the northernmost populations showed no loss of rare alleles. Patterns of mitotype diversity were strikingly different: within-population diversity and population differentiation were high for boreal forest populations [expected heterozygosity per locus ( H E ) = 0.58 and θ M = 0.529], but all subarctic populations were fixed for a single mitotype ( H E = 0). This lack of variation suggests a founder event caused by long-distance seed establishment during postglacial colonization, consistent with palaeoecological data. The estimated movement of seeds alone (effective number of migrants per generation, Nm M 17). This could account for the conservation of a founder imprint in the mtDNA of subarctic black spruce. After reduction, presumably in the early Holocene, the diversity in ncDNA would have been replenished rapidly by pollen-mediated gene flow, and maintained subsequently through vegetative layering during the current cooler period covering the last 3000 years.

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Diverging patterns of mitochondrial and nuclear DNA diversity in subarctic black spruce: imprint of a founder effect associated with postglacial colonization.

Phylogenetic relationships, possible ancient hybridization, and biogeographic history of Abies (Pinaceae) based on data from nuclear, plastid, and mitochondrial genomes.

Patterns of nuclear and organelle inheritance among artificial hybrids of the seaweeds Fucus serratus and F. evanescens were detected using single-strand conformation polymorphism (SSCP). Three alleles were identified in the 231 bp rDNA-ITS1 gene (nuclear): two in F. serratus and one in F. evanescens. Alleles differed by 1–2 bp and all hybrids possessed one allele from each parent. Two haplotypes were present in the 288 bp Rubisco spacer (chloroplast), differentiated by a 33 bp indel. Two haplotypes differing by a single nucleotide were found in a 135 bp region of nad11 gene (mitochondrion). Both organelles are maternally inherited, as all hybrids contained the haplotypes of the parent contributing the egg. Although laboratory hybrids among Fucus spp. have been produced previously, this is the first time that both nuclear and cytoplasmic genetic markers have been used to document inheritance patterns. SSCPs analysed on an automated sequencer offer a rapid and powerful approach for identifying suspected hy...

Inheritance patterns of ITS1, chloroplasts and mitochondria in artificial hybrids of the seaweeds Fucus serratus and F. evanescens (Phaeophyceae)

Sub-alpine Abies veitchii and A. homolepis are distributed in the central part of Honshu Island, Japan, and their habitats are segregated vertically. These species sometimes form a mixed forest in the overlapping area of the two species, that is, in the upper limit of the A. homolepis habitat and the lower limit of A. veitchii. These species have been considered to be distantly related because they were classified into different sections by most conventional classifications. No natural hybridization has been reported between the two species. The aim of this study was to demonstrate, through the use of molecular markers, whether natural hybridization takes place between these two species at two experimental sites on Mt. Fuji, where the species occur naturally. DNA markers from paternally inherited chloroplast DNA (cpDNA), maternally inherited mitochondrial DNA (mtDNA) and biparentally inherited nuclear DNA (nDNA), were used for this study. As organelle DNA markers, polymerase chain reaction-single strand conformation polymorphism (PCR-SSCP) markers were developed to determine the maternal and paternal species for each individual. Two of 334 individuals possessed a cpDNA haplotype derived from A. homolepis and a mtDNA haplotype from A. veitchii. Furthermore, the nDNA of these two individuals was analysed using the random amplified polymorphic DNA (RAPD) assay to investigate their genomic composition. RAPD analysis indicated that the nuclear genomes of the two individuals were derived from both species. We conclude that A. veitchii and A. homolepis produce natural hybrids, and that their systematic relationship should be re-evaluated.

Molecular evidence of natural hybridization between Abies veitchii and A. homolepis (Pinaceae) revealed by chloroplast, mitochondrial and nuclear DNA markers.

K. Kitamura

Papers

Molecular evidence of natural hybridization between Abies veitchii and A. homolepis (Pinaceae) revealed by chloroplast, mitochondrial and nuclear DNA markers.