Some notes about Calochortus nudus, C. minimus, and hybrids

Two closely related species of Calochortus occur in the mountains of California and southern Oregon; C. nudus in the Klamath Range/Mt Shasta area and C. minimus in the Sierra Nevada from Lake Tahoe southward. In between the areas where the species occur in their pure form is a large region covered by intermediate types which are believed to be derived from hybridization and introgression with the parent species.

Marion Ownbey described the situation in his 1940 Monograph of the Genus Calochortus (https://www.biodiversitylibrary.org/page/16163357). He first described C. minimus in this paper, and chose to assign the intermediate forms to C. nudus:

In the vicinity of Mount Shasta, C. nudus is uniform, but south of the Pit River it occurs in pure stand with decreasing frequency as one passes southward. In eastern Eldorado County and southward, only the closely related C. minimus occurs. Between the two geographically, there is a bewildering assortment of plants showing independent recombination of the various morphological characters which separate these two species. Such a population can be explained only as the result of long-continued hybridization and probably repeated back-crossing, particularly with C. nudus. Pure C. minimus does not seem to occur within the area, so the entire population is here referred to C. nudus. It should be pointed out, however, that occasional specimens are so close to C. minimus that they can be distinguished only by geographical criteria.
From the evidence at hand, it appears that at one time these species were separated by a geographical barrier which allowed evolution to proceed in different directions on either side. As a result there was developed a robust northern race, with larger flowers, rounded petals, taller stems, proportionately shorter and broader basal leaves, and erect fruits which are acute at both ends. It is fortunate that this race has persisted in a nearly pure state in the Mount Shasta Region, and at numerous stations in the northern Sierra Nevada.

The southern race is smaller in all respects, the petals acute, the stems very short, the basal leaves greatly exceeding the inflorescences, the fruits obtuse and nodding on slender, strongly deflexed pedicels. This race now occupies the southern Sierra Nevada, from eastern Eldorado County southward to Tulare County, in a practically pure condition. The combinations of morphological criteria which separate the southern from the northern race are certainly of specific value. It is only when the intervening population is considered that there is any possibility of another interpretation.

Today the barrier which once separated these two species has disappeared, and they have come together again. Since they were presumably derived from the same stock, the hybrids are fertile and interbreed both among themselves and with both parent species. The result should be a population possessing the characters of both parents, but in different combinations. This is exactly what we find. It is impossible to separate such a population completely into two, or even a dozen, categories, yet the morphological differences between C. nudus and C. minimus do not permit their inclusion within a single species. Even if such an assignment were possible, it would be undesirable, as it would obscure their probable genetic relationships.

Regional floras have followed Ownbey, for example "A Flora of Lassen Volcanic National Park, California" (Gilllett et al, 1995) lists only C. nudus as occurring in the park.

Fifty years later Ness et al (1990) carried out additional studies which supported the hypothesis that the intermediate forms in the northern Sierra result from hybridization. Their research suggested that the hybrids are no more closely related to C. nudus than to C. minimus. (https://www.researchgate.net/publication/230852138_An_Examination_of_Polyploidy_and_Putative_Introgression_in_Calochortus_Subsection_Nudi_Liliaceae)

Patterson and Givnish (2003) conducted DNA analysis to study evolution of Calochortus. Although they do not specifically discuss C. nudus and C. minimus, their analysis indicates that these two are not sister taxa; that is, each is more closely related to other species of Calochortus that they are to each other. (https://courses.botany.wisc.edu/botany_940/15Stebbins/chapter%206%20papers/Patterson%202004%20New%20Phytologist.pdf)

Posted on July 22, 2019 11:25 PM by