Mimulus peregrinus (Phrymaceae): A new British allopolyploid species

Abstract

Polyploidization plays an important role in species formation as chromosome doubling results in strong reproductive isolation between derivative and parental taxa. In this note I describe a new species, Mimulus peregrinus (Phrymaceae), which represents the first recorded instance of a new British polyploid species of Mimulus (2n = 6x = 92) that has arisen since the introduction of this genus into the United Kingdom in the 1800's. Mimulus peregrinus presents floral and vegetative characteristics intermediate between Mimulus guttatus and Mimulus luteus, but can be distinguished from all naturalized British Mimulus species and hybrids based on a combination of reproductive and vegetative traits. Mimulus peregrinus displays high pollen and seed fertility as well as traits usually associated with genome doubling such as increased pollen and stomata size. The intermediate characteristics of Mimulus peregrinus between Mimulus guttatus (2n = 2x = 28)and Mimulus luteus (2n = 4x = 60-62), and its close affinity with the highly sterile, triploid (2n = 3x = 44-45) hybrid taxon Mimulus × robertsii (Mimulus guttatus × Mimulus luteus), suggests that Mimulus peregrinus mayconstitute an example of recent allopolyploid speciation.

Keywords: Allopolyploidy; Erythranthe; Mimulus guttatus; Mimulus luteus; hybrid evolution; introduced species; rapid evolution; speciation.

Figure 1.

Holotype of Mimulus peregrinus Vallejo-Marin [11-LED-seed-2-14; barcode E00570050].

Figure 2.

Flowers of Mimulus peregrinus and closely related taxa. A Mimulus guttatus B Mimulus × smithii (Mimulus luteus luteus × Mimulus luteus variegatus) C Mimulus × robertsii (Mimulus guttatus × Mimulus luteus), and D Mimulus peregrinus. Each taxon is represented by flowers from two individuals from a single locality to illustrate within-population variability: Mimulus guttatus = Dunblane, Perthshire; Mimulus × smithii = Coldstream, Scottish Borders; Mimulus × robertsii = Nenthall, Cumbria; Mimulus peregrinus = Leadhills, South Lanarkshire. Scale bar = 1cm.

Figure 3.

Flow-cytometry estimates of 2C DNA content (DAPI-stained) of British Mimulus. Error bars represent standard errors when multiple individuals per taxon were tested. Sample sizes as follows (chromosome numbers for each population are given in parenthesis when available). Mimulus guttatus: N = 4 individuals from Dunblane, Perthshire (2n = 28); and 2 individuals from Muckle Roe, Shetland; Mimulus × robertsii (= Mimulus guttatus × Mimulus luteus): N= 1 individual from Nenthall, Cumbria (2n = 44, 45); Mimulus × smithii (= Mimulus luteus var. luteus × Mimulus luteus var. variegatus): N = 2 individuals from Coldstream, Scottish Borders (2n = 59, 60, 61, 62); Mimulus peregrinus: N = 6 individuals from Leadhills, South Lanarkshire (2n = 92).All chromosome counts kindly provided by J. Bailey.