Patterns of evolution in western North American Mimulus (Phrymaceae)

Abstract

A well-supported phylogeny is presented from both chloroplast DNA (the trnL/F region) and two regions of nuclear rDNA (ITS [internal transcribed spacer] and ETS [external transcribed spacer]) with nearly complete sampling for Mimulus (Phrymaceae) in western North America. Three separate genera are derived from within the clade that contains all the Mimulus species in western North America. The taxonomic status of the proposed sections of Mimulus and the relationships of many taxonomically difficult species are considered with observations on morphological evolution. Discordance between data sources provides support for the hypothesis that M. evanescens is a hybrid between M. latidens and M. breviflorus. In two major clades (Eunanus and Diplacus), patterns of genetic variation do not match the current taxonomy. The clustering of taxa in Eunanus is strongly associated with geographic distributions. Mimulus aurantiacus sensu Thompson, M. nanus, and M. floribundus are found to be progenitor species to other species that appear to be derived from within them. Polyploidy and aneuploidy events are clustered near the tips of the phylogeny. Thus, these two mechanisms are concluded to have played a relatively small role in the evolution of persistent lineages in Mimulus. The phylogenetic distribution of rare taxa is also examined.

Comparative linkage maps suggest that fission, not polyploidy, underlies near-doubling of chromosome number within monkeyflowers (Mimulus; Phrymaceae)

“…In this case, the lower chromosome numbers of Erythranthe would be secondarily derived by descending aneuploidy and homoplasic with the other n ¼ 8 taxa. However, analyses of expressed sequence tag libraries from M. lewisii and M. guttatus indicate that these taxa share a most recent whole-genome duplication event about 46-70 Mya (Clarke, 2012), which pre-dates the estimated origins of both the genus Mimulus (35 Mya) and the family Phrymaceae (40 Mya) (Beardsley et al, 2004;Nie et al, 2006). It is also theoretically possible that Paradanthus and Simiolus retain ancestral high chromosome numbers from this ancient polyploidization, and that all Mimulus clades with lower chromosome numbers Figure 1 Synteny between concatenated M. guttatus (GIM 1-14; colors) and M. lewisii (LPC 1 þ 8-7; black) linkage maps.…”

“…Chromosome number evolution in monkeyflowers L Fishman et al have separately undergone reduction in chromosome number by fusion (in a minimum of four distinct lineages; Beardsley et al, 2004). In this case, the eight haploid chromosomes of Erythranthe taxa would result from recent (o20 Mya) fusion, and would again not correspond to the chromosomes of other low-number taxa.…”

“…The genus, which has its center of diversity and likely origin in western North America, has traditionally been divided into seven sections. Two sections, Simiolus and Paradanthus, consist of primarily yellow-flowered taxa and have n ¼ 14 and 16, respectively, as the common haploid chromosome numbers (Beardsley et al, 2004). Most of the remaining Mimulus sections, including the section Erythranthe, have a common haploid chromosome number of 8 (n ¼ 9-10 in a few cases).…”

“…Most of the remaining Mimulus sections, including the section Erythranthe, have a common haploid chromosome number of 8 (n ¼ 9-10 in a few cases). Paradanthus appears polyphyletic, with one clade sister to Simiolus and the other to Erythranthe (Beardsley et al, 2003(Beardsley et al, , 2004. However, these three sections together form a monophyletic group contained within the rest of Mimulus (Beardsley et al, 2004), and share a most recent common ancestor about 20 million years ago (Mya) (Nie et al, 2006).…”

“…However, these three sections together form a monophyletic group contained within the rest of Mimulus (Beardsley et al, 2004), and share a most recent common ancestor about 20 million years ago (Mya) (Nie et al, 2006). Because phylogenetic reconstructions strongly suggests that n ¼ 8 is ancestral in Mimulus, the B2-fold difference in chromosome number between Erythranthe species (and most other taxa) and the yellow monkeyflowers has been inferred to represent a whole-genome duplication event specific to Simiolus and its sister clade of yellow Paradanthus (Figure 4 of Beardsley et al, 2004).…”