Chromosome evolution in flowering plants is often punctuated by polyploidy, genome duplication events that fundamentally alter DNA content, chromosome number, and gene dosage Polyploidy confers postzygotic reproductive isolation and is thought to drive ecological divergence and range expansion The adaptive value of polyploidy, however, remains uncertain; ecologists have traditionally relied on observational methods that cannot distinguish effects of polyploidy per se from genic differences that accumulate after genome duplication Here I use an experimental approach to test how polyploidy mediates ecological divergence in Achillea borealis (Asteraceae), a widespread tetraploid plant with localized hexaploid populations In coastal California, tetraploids and hexaploids occupy mesic grassland and xeric dune habitats, respectively Using field transplant experiments with wild-collected plants, I show that hexaploids have a fivefold fitness advantage over tetraploids in dune habitats Parallel experiments with neohexaploids—first-generation mutants screened from a tetraploid genetic background—reveal that a 70% fitness advantage is achieved via genome duplication per se These results suggest that genome duplication transforms features of A borealis in a manner that confers adaptation to a novel environment

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Polyploidy and ecological adaptation in wild yarrow

Adaptive divergence due to habitat differences is thought to play a major role in formation of new species. However it is rarely clear the extent to which individual reproductive isolating barriers related to habitat differentiation contribute to total isolation. Furthermore, it is often difficult to determine the specific environmental variables that drive the evolution of those ecological barriers, and the geographic scale at which habitat-mediated speciation occurs. Here, we address these questions through an analysis of the population structure and reproductive isolation between coastal perennial and inland annual forms of the yellow monkeyflower, Mimulus guttatus. We found substantial morphological and molecular genetic divergence among populations derived from coast and inland habitats. Reciprocal transplant experiments revealed nearly complete reproductive isolation between coast and inland populations mediated by selection against immigrants and flowering time differences, but not postzygotic isolation. Our results suggest that selection against immigrants is a function of adaptations to seasonal drought in inland habitat and to year round soil moisture and salt spray in coastal habitat. We conclude that the coast and inland populations collectively comprise distinct ecological races. Overall, this study suggests that adaptations to widespread habitats can lead to the formation of reproductively isolated species.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1558-5646.2008.00457.x

Ecological reproductive isolation of coast and inland races of mimulus guttatus

The genetic basis of species differences provides insight into the mode and tempo of phenotypic divergence. We investigate the genetic basis of floral differences between two closely related plant taxa with highly divergent mating systems, Mimulus guttatus(large-flowered outcrosser) and M. nasutus(small-flowered selfer). We had previously constructed a framework genetic linkage map of the hybrid genome containing 174 markers spanning approximately 1800 cM on 14 linkage groups. In this study, we analyze the genetics of 16 floral, reproductive, and vegetative characters measured in a large segregating M. nasutus 3 M. guttatus F2 population (N 5 526) and in replicates of the parental lines and F1 hybrids. Phenotypic analyses reveal strong genetic correlations among floral traits and epistatic breakdown of male and female fertility traits in the F2 hybrids. We use multitrait composite interval mapping to jointly locate and characterize quantitative trait loci (QTLs) underlying interspecific differences in seven floral traits. We identified 24 floral QTLs, most of which affected multiple traits. The large number of QTLs affecting each trait (mean 5 13, range 5 11-15) indicates a strikingly polygenic basis for floral divergence in this system. In general, QTL effects are small relative to both interspecific differences and environmental variation within genotypes, ruling out QTLs of major effect as contributors to floral divergence between M. guttatus and M. nasutus. QTLs show no pattern of directional dominance. Floral characters associated with pollinator attraction (corolla width) and self-pollen de- position (stigma-anther distance) share several pleiotropic or linked QTLs, but unshared QTLs may have allowed selfing to evolve independently from flower size. We discuss the polygenic nature of divergence between M. nasutus and M. guttatus in light of theoretical work on the evolution of selfing, genetics of adaptation, and maintenance of variation within populations.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.0014-3820.2002.tb00139.x

Minor quantitative trait loci underlie floral traits associated with mating system divergence in Mimulus.

Chloroplast trnL/F and nuclear ribosomal ITS and ETS sequence data were used to analyze phylogenetic relationships among members of tribe Mimuleae (Scrophulariaceae) and other closely related families in Lamiales. The results of these analyses led to the following conclusions. (1) The Australian genera Glossostigma and Peplidium and the taxonomically isolated Phryma join four genera of tribe Mimuleae to form a well-supported clade that is distinct from other families in the Lamiales. We refer to that clade as the subfamily Phrymoideae. (2) The genera Mazus and Lancea (tribe Mimuleae) together form a well-supported clade that we recognize as the subfamily Mazoideae. Mazoideae is weakly supported as sister to Phrymoideae. We assign Mazoideae and Phrymoideae to a redefined family Phrymaceae. (3)Mimulus is not monophyletic, because members of at least six other genera have been derived from within it. In light of the molecular evidence, it is clear that species of Phrymaceae (about 190 species) have undergone two geographically distinct radiations; one in western North America (about 130 species) and another in Australia (about 30 species). Phylogenetic interpretations of morphological evolution and biogeographical patterns are discussed.

Redefining Phrymaceae: the placement of Mimulus, tribe Mimuleae, and Phryma.

Evolutionary biologists seek to understand the genetic basis for multivariate phenotypic divergence. We constructed an F2 mapping population (N = 539) between two distinct populations of Mimulus guttatus. We measured 20 floral, vegetative, and life-history characters on parents and F1 and F2 hybrids in a common garden experiment. We employed multitrait composite interval mapping to determine the number, effect, and degree of pleiotropy in quantitative trait loci (QTL) affecting divergence in floral, vegetative, and life-history characters. We detected 16 QTL affecting floral traits; 7 affecting vegetative traits; and 5 affecting selected floral, vegetative, and life-history traits. Floral and vegetative traits are clearly polygenic. We detected a few major QTL, with all remaining QTL of small effect. Most detected QTL are pleiotropic, implying that the evolutionary shift between these annual and perennial populations is constrained. We also compared the genetic architecture controlling floral trait divergence both within (our intraspecific study) and between species, on the basis of a previously published analysis of M. guttatus and M. nasutus. Eleven of our 16 floral QTL map to approximately the same location in the interspecific map based on shared, collinear markers, implying that there may be a shared genetic basis for floral divergence within and among species of Mimulus.

https://www.genetics.org/content/genetics/172/3/1829.full.pdf

Pleiotropic Quantitative Trait Loci Contribute to Population Divergence in Traits Associated With Life-History Variation in Mimulus guttatus

THE region of the Pacific States with which the book under review deals is of great interest to the student of geographical distribution and the taxonomist. The area is peculiarly rich in endemic genera and species, and this is especially true of California. The Pacific States show interesting geographical connexions with the flora of Eastern Asia in genera, however, which are probably too recent in origin to be adduced in support of any hypothesis of continental drift, but are to be regarded rather as evidence of the similarity of ecological conditions and the high efficiency of dispersal agents, not perhaps excluding man himself. Thus the saxifragaceous species Leptarrhena pyrolifolia is native both to the coastal regions of north-west America and to eastern Asia. Again, the genus Pectiantia comprises five species of which three are endemic to the Pacific States, whereas the other two are natives of Japan. But it is in the endemics of the region that the chief interest of the flora rests. Some of these, such as Romneya Coulteri, the genera Eschscholtzia and Lewisia, are familiar to British horticulturists. In the family Polygonaceae the monotypic genera Gilmania, Nemacaulis and Hollisteria are all endemic to California while of the 150 known species of Eriogonum 80 are found in the Pacific States, many of them endemic. In the Crassulaceae there are some 35 endemic species belonging to the genera Sedella, Budleya, Stylophyllum, Hasseanthus and Gormania. In the Cruciferse most of the species of Thelypodium, Streptanthus and Caulanthus are endemic. The Rosacese, too, provide a large number of endemics, particularly in the genera Horskelia (19 spp.) and Ivesia (17 spp.). In the Leguminosse the outstanding genera in this respect are Lupinus, of which 58 species are probably endemic, Hosackia, of which most of the 39 species are endemic, while Astragalus is represented by 137 species of which 83 are probably endemic. Illustrated Flora of the Pacific States Washington, Oregon and California. By Leroy Abrams. In 4 vols. Vol. 2: Polygonaceae to Krameriaceae, Buckwheats to Kramerias. Pp. viii + 635. (Stanford University, Calif.: Stanford University Press; London: Oxford University Press, 1944.) 45s. 6d. net.

E. J. Salisbury

Papers

Illustrated Flora of the Pacific States