Mapping Species Distributions

Small-flowered blue-eyed Mary (Collinsia parviflora), photo by Gary Ansell.

Wildflower Genetics: Purple Spotted Leaves in
Blue-eyed Mary (Collinsia parviflora)

by
Anthony J. F. Griffiths and Fred R. Ganders

Some populations of blue-eyed Mary in southern British Columbia are polymorphic for a gene that causes purple-spotted leaves. The spots result from patches of cells in the upper epidermis of the leaf that contain anthocyanin pigment. Leaves may show heavy, medium, or very faint spotting, or have no spots at all. The presence or absence of spots is determined by a single gene, with the allele for spots exhibiting dominance. The very faint spots, however, may be caused by a separate gene or by a different allele of the spot gene.

The leafs spots in blue-eyed Mary are conspicuous only in seedlings and young plants. They tend to fade as the plants get older and may also be masked by other anthocyanin pigments that suffuse the entire plant as it ages. In addition, the expression of the spot gene is affected by the environment. The spotted phenotype is expressed only at relatively low temperatures, below 20C. If plants with spotted genotypes are grown at higher temperatures, the spots are not expressed. If spotted plants are moved from a cool to a warm environment, such as a greenhouse, the spots disappear in a few days. Spotted leaves in blue-eyed Mary are a good example of how both the genotype and the environment interact to produce the phenotype, or appearance, exhibited by the individual plant.

Spotted blue-eyed Marys are not randomly distributed in populations in British Columbia. The leaf-spot gene has been found only in populations in the central part of Vancouver Island, near the Strait of Georgia, and on islands in the Strait. The populations on southern Vancouver Island and the Lower Mainland are monomorphic for spotless plants. On the Flat Top Islands, a group of small islands east of Gabriola Island and southeast of Nanaimo, the distribution of the spotted phenotype shows a pattern that is also reflected by spotted genes in annual clover (Trifolium willdenowii) and yellow monkey flower (Mimulus guttatus). The fact that all three species show high frequencies of spots in the same site and low frequencies when growing together in other sites strongly suggests that some environmental factor is selecting for the presence or absence of the leaf-spot gene.

The situation on Carlos Island is particularly instructive. Carlos Island is a small, low island at the northern end of the Flat Top Islands, and its north side receives the full force of cold winter winds blowing down the Strait of Georgia. The south side is relatively protected by wind-trimmed trees, and the gentle south slope is much warmer. The frequency of the spotted-leaf gene is much higher on the cold north side than on the warmer south side. This suggests that the spotted leaves are favoured in colder environments. One reason may be that the dark colour absorbs sunlight and heats up the leaf more effectively and that higher leaf temperatures increase the rate of photosynthesis in the plants during the winter. Blue-eyed Mary, as well as clover and monkey-flower, begin growing in the fall and grow slowly through the winter, forming rosettes of leaves near the soil surface. They begin flowering in March or April, and, on the Flat Top Islands, they die by June or early July.

Reprinted from Anthony J. F. Griffiths and Fred R. Ganders. 1983. Wildflowers Genetics: A Field Guide for British Columbia and the Pacific Northwest.
Flight Press, Vancouver--with permission.

This publication is available for purchase form the UBC Botanical Garden gift shop.