Torrey’s yucca, Yucca torreyi, is one of two yuccas that we have in our area. I was out botanizing yesterday south of Abilene (Texas,USA) in Abilene State Park and found a good number of these monocots in full flower. Torrey’s Yucca is a member of the plant family Agavaceae. The plant pictured may be a little non-typical in that the panicle inflorescence, which is the entire mass of flowers, is not sitting up higher in comparison to the long leaves. Typically, the upper half of the panicle of flowers will stand above the top leaves. However, the length of the sword-like leaves on this particular plant are about 90cm, which is typical of this species. The other species of yucca found here on the Rolling Plains, Yucca constricta, has leaves no longer than about 65cm. One final supporting morphological feature found on the leaves of this specimen are the narrow threads on the margins.
Below is a close up image of the inflorescence. Note the large number of pendant or drooping flowers.
Below, a close up of the inside of one flower:
To the best of my knowledge the yuccas (species in the genus Yucca) are endemic only to North America with 35 species total in the genus. They are found in warm areas and were used extensively by Native Americans as a source of food, fiber, soap, and medicine. Torrey’s Yucca is named for John Torrey (1796-1873), a distinguished American botanist, physician, and collector of many western North American plants.
Pollination in yuccas is interesting. All species are dependent on the yucca moth for pollination. Apparently, if the moths are not present Yuccas can reproduce vegetatively (which is a nice evolutionary adaptation to have). Quoting from Powell (1988):
The yucca moth (Tegeticula = Pronuba) flies at dusk to a flower where she climbs stamens to collect pollen and pack the pollen in a large ball-like mass under her neck. She then visits another flower where she inserts her ovipositer directly through the ovary wall and deposits 20-30 eggs, one at a time, each directly in to an ovule. She then climbs to the stigma of the same flower and spreads the pollen, thus ensuring pollination, subsequent fertilization, and developing seeds that provide nourishment for the moth larvae. Each larva ultimately destroys the seed in which it grows, but there are many undamaged seeds left in the yucca capsule.
From Google a picture of a yucca moth (http://en.wikipedia.org/wiki/Prodoxidae):
A species of yucca moth in the family Prodoxidae
The relationship between the yucca and yucca moth is termed a mutualism, in a general ecological context. More specifically, this is a marvelous example of an obligate mutualism for the moth species of the genera Tegeticula, which both pollinates and deposits eggs in Yucca flowers. For the moth, the relationship to yucca is termed an obligate mutualism because the moth larvae feed only on yucca seeds. The yucca moth has special mouth parts which allow it to package and transfer the flower pollen. For the yucca plant the relationship is mostly obligate because only yucca moths transfer their pollen between yucca flowers for fertilization. I say mostly obligate since yuccas can reproduce vegetatively, so they can perpetuate themselves without sexual reproduction. There are advantages and disadvantages to asexual reproduction. However, the bottom line for yuccas is that they have coevolved with the yucca moth to form a very unique mutualistic relationship. Therefore, it’s probably fairly safe to say that sexual reproduction via the yucca moth is most likely the primary means of reproduction for the yucca. Such a highly evolved relationship with an insect is not happenstance.
The evolutionary history of the other genera of the moth family Prodoxidae is quite fascinating. A second genus in this moth family, Parategeticula, also has an obligate relationship with yucca flowers as well. Individuals of a third genus of yucca moths, Prodoxus, do not have a mutualistic relationship with yucca plants, neither depositing their eggs in yucca seeds nor moving around pollen from the flowers. However, Prodoxus eggs are deposited in the fruits (mature ovary from a flower) and leaves of yuccas where they eat and grow, which is more of a herbivorous relationship.
Hopefully, I have all of this evolutionary history straight. I have a huge interest in plant-animal coevolution and, as an evolutionary botanist, delving into a story such as this is like dining at a five-star restaurant (minus the huge bill at the end!). A reference for those wanting more is the article by Pellmyr et al. (1996), referenced below. This is recommended reading for those who find this plant-moth saga as fascinating as I do.
One never knows where an afternoon of botanizing will lead….
Powell, A.M., 1988. Trees and shrubs of Trans-Pecos Texas including Big Bend and Guadalupe Mountains National Parks. Big Bend Natural History Association.
Pellmyr, O., J. N. Thompson, J. Brown, and R. G. Harrison. 1996. Evolution of pollination and mutualism in the yucca moth lineage. American Naturalist. 148: 827–847.
Copyright © Rick L. Hammer 2009