Changes in chromosome number and structure are important contributors to adaptation, speciation and macroevolution. In flowering plants, polyploidy and subsequent reductions in chromosome number by fusion are major sources of chromosomal evolution, but chromosome number increase by fission has been relatively unexplored. Here, we use comparative linkage mapping with gene-based markers to reconstruct chromosomal synteny within the model flowering plant genus Mimulus (monkeyflowers). Two sections of the genus with haploid numbers X14 have been inferred to be relatively recent polyploids because they are phylogenetically nested within numerous taxa with low base numbers (n ¼ 8-10). We combined multiple data sets to build integrated genetic maps of the M. guttatus species complex (section Simiolus, n ¼ 14) and the M. lewisii group (section Erythranthe; n ¼ 8), and then aligned the two integrated maps using 4100 shared markers. We observed strong segmental synteny between M. lewisii and M. guttatus maps, with essentially 1-to-1 correspondence across each of 16 chromosomal blocks. Assuming that the M. lewisii (and widespread) base number of 8 is ancestral, reconstruction of 14 M. guttatus chromosomes requires at least eight fission events (likely shared by Simiolus and sister section Paradanthus (n ¼ 16)), plus two fusion events. This apparent burst of fission in the yellow monkeyflower lineages raises new questions about mechanisms and consequences of chromosomal fission in plants. Our comparative maps also provide insight into the origins of a chromosome exhibiting centromere-associated female meiotic drive and create a framework for transferring M. guttatus genome resources across the entire genus.
Across a species range, islands of stressful habitats impose similar selection pressures on isolated populations. It is as yet unclear, when populations respond to these selective pressures, the extent to which this results in convergent genetic evolution and whether convergence is due to independent mutations or shared ancestral variation. We address these questions investigating a classic example of adaptation by natural selection - the colonization of plant species to heavy metal contaminated soils. We use field-based reciprocal transplant experiments to demonstrate that mine alleles at a major copper tolerance QTL, Tol1, are strongly selected in the mine environment, but are neutral, or nearly so, in the off-mine environment. We assemble the genome of a mine adapted genotype and identify regions of this genome in tight genetic linkage to Tol1. We discover Tol1 candidate genes that exhibit significantly large differences in expression between tolerant and non-tolerant genotypes or in allele frequency between mine/off-mine population pairs. We identify a single gene, a multicopper oxidase, which exhibits both large differences in expression and allele frequency. Furthermore, patterns of genetic variation at the five loci with the greatest difference in allele frequency between populations, including the multicopper oxidase, are consistent with selection acting upon beneficial haplotypes that predates the existence of the copper mine habitat. We estimate the age of selected Tol1 haplotype to be at least 1700 years old and was at a frequency of 0.4-0.6% in the ancestral population when mining was initiated 150 years ago. These results suggest that adaptation to the mine habitat routinely occurs via selection on ancestral variation, rather than independent de-novo mutations or migration between populations.
of species in the genera of the Ceylon flora, a curve which soon proved to be universal in both floras and faunas. This led to the development of the theory set out in Age and Area in 1922. Being, among other things, a flat contradiction of the theory of gradual adaptation through the agency of natural selection, this theory of age and area was not accepted, but as the counter viii PREFACE arguments brought up mostly assumed that the older theory was sound, the writer's faith remained unchanged, and he continued to follow up his beliefs. They are now yielding interesting results, of which the present book is one, while another, dealing with distribution, and whichis perhaps even more subversive of current opinions (used as a shelter for so much in national policies), is upon the road to completion. The present book, the logical sequence of Age and Area, has been greatly delayed by various inconveniences, and by the great quantity of statistical work required. This was so great a burden that I can hardly sufficiently express my gratitude to my friend Mr John Murray, late of the Indian Educational Department, who undertook a great deal of it, and with his trained mathematical skill was able to do it well and rapidly. I am also deeply
11Across western North America, Mimulus guttatus exists as many local populations adapted to site-12 specific challenges including salt spray, temperature, water availability, and soil chemistry. Gene flow 13 between locally adapted populations will effect genetic diversity in both local demes and across the larger
Polymorphic chromosomal rearrangements, which can bind together hundreds of genes into single genetic loci with diverse effects, are increasingly associated with local adaptation and speciation. They may also be an important component of genetic variation within populations.We genetically and phenotypically characterized a novel segregating inversion (inv6) in the Iron Mountain (IM) population of Mimulus guttatus (yellow monkeyflower). We first identified a region of recombination suppression in three F 2 mapping populations resulting from crosses among IM plants; in each case, the F 1 hybrid parent was heterozygous for a homogenous derived haplotype (inv6) across markers spanning over 4.2 Mb of Linkage Group 6. Genotype-phenotype associations in the three F 2 populations demonstrated negative inv6 effects on male and female fitness components. In addition, inv6 carriers suffered a ~30% loss of pollen viability in the field. Despite these costs, inv6 exists at moderate and apparently stable frequency (~7%) in the natural population, suggesting counter-balancing fitness benefits that maintain the polymorphism. Across four years of monitoring in the field, inv6 had an overall significant positive effect on the seed production (lifetime female fitness) of carriers. This benefit was particularly strong in harsh years and may be mediated (in part) by strong positive inv6 effects on flower production. These data suggest that opposing fitness effects maintain an intermediate frequency, and as a consequence, inv6 generates inbreeding depression and high genetic variance. We discuss these findings in the context of theory about the genetic basis of inbreeding depression and the role for chromosomal rearrangements in population divergence with gene flow.peer-reviewed) is the author/funder. All rights reserved. No reuse allowed without permission.The copyright holder for this preprint (which was not . http://dx.doi.org/10.1101/028670 doi: bioRxiv preprint first posted online Oct. 8, 2015; 3 Keywords (5) inbreeding depression, polymorphism, variation, chromosomal rearrangement, Mimulus guttatus Summary (100 words)Polymorphic chromosomal rearrangements may contribute importantly to genetic variation within populations. Here, we identify a novel segregating inversion (inv6) in that contributes to fitness variation in a natural population of Mimulus guttatus (yellow monkeyflower).Greenhouse studies demonstrate negative inv6 effects on male and female fitness components.Despite these costs, inv6 exists at moderate and apparently stable frequency (~8%) in nature, and four years of field data, indicate significant positive effects on female fecundity. Opposing fitness effects apparently maintain an intermediate frequency, and as a consequence, inv6 generates substantial genetic variance in fitness.peer-reviewed)
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