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3-2012 Partridge Berry: Simple Beauty Belies Complexity W. John Hayden University of Richmond, [email protected]

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Recommended Citation Hayden, W. John. "Partridge Berry: Simple Beauty Belies Complexity." Bulletin of the Virginia Native Plant Society 31, no. 1 (March 2012): 4 ,8.

This Article is brought to you for free and open access by the Biology at UR Scholarship Repository. It has been accepted for inclusion in Biology Faculty Publications by an authorized administrator of UR Scholarship Repository. For more information, please contact [email protected]. Bulletin of the Virginia Native Plant Society Partridge berry: WOY 2012 Simple beauty belies complexity Superficially, seem so and long forms of both simple. Rooted in place, they do not move styles and .) By around. And while plant growth is a dy- convention, long style namic process, without time-lapse pho- flowers are called “pin” tography, growth events are so imper- flowers, which is descrip- ceptibly slow that, to us impatient hu- tive of flowers like prim- mans, plants seem both immobile and roses in which the stigma static. Nevertheless, there is a lot going resembles a round-headed on inside the plant body, and this is es- pin; long /anther pecially true for the events of reproduc- flowers are known as tion that play out inside flowers and “thrums,” an obscure ref- fruits. As one of my students recently erence to the ragged ends commented, “I used to think it was just a of threads protruding from matter of pollen plus stigma and, presto- woven cloth. Despite the Pin and thrum flower forms of repens, change-o, seeds happen.” That stu- fact that the four flap-like partridge berry; redrawn from Ganders 1975. dent, I hope, learned otherwise, as will stigmas of partridge berry in no way distylous flowers like partridge berry. anyone else who takes the time to study resemble pin-heads, for consistency As it turns out, there are genes govern- the biology of flowers in detail. with the terminology applied to other ing self-incompatibility interactions at Take, for example, the 2012 Wild- species, long-style partridge berry flow- the cellular and molecular level that flower of the Year, partridge berry ers are still termed pins (see figure). cause pollen tubes to abort, and these (Mitchella repens). Flowers appear in So, what is the point of distylous genes are tightly linked with the genes late spring and continue somewhat floral heteromorphy? As it turns out, that control stamen and style length. sporadically through the summer. In the two different floral forms are part The details of how self-incompatibil- any given patch of partridge berry, it is of a system of adaptations that control ity works varies from one group of most likely that all the flowers will pollination, and hence, the subsequent plants to another, but regardless of the appear identical. But if one carefully fertilization of ovules that, in turn, im- details, self-incompatibility genes are examines flowers from multiple colo- pacts the genetic composition of the usually denoted by the symbol S. nies, it will be apparent that this spe- seeds produced. The way the system In the most generalized example cies produces two different flower works is that pollen from long stamens of how these systems work, the self-in- forms (see figure). In other words, the with protruding anthers (thrums) func- compatibility gene has numerous al- flowers are heteromorphic. In some tions only on flowers with long styles ternative forms (alleles) designated as flowers, stigmas protrude beyond the and protruding stigmas (pins); con- S1, S2, S3, . . . Sn. These alleles are ex- corolla while anthers are hidden versely, pollen from pin flowers can pressed by the production of certain within the corolla tube. In other flow- function only on thrum stigmas. Self- proteins, both in the cells of the style ers, the pattern is reversed, stamens are pollination fails, as does thrum pollen and stigma and in the pollen grains. long with protruding anthers and on stigmas of other thrum flowers, and Because the floral heteromorphism styles are short, with the stigmas hid- pin pollen on stigmas of other pin flow- genes are tightly linked to the incom- den inside the corolla tube. This par- ers. Cross-pollination, of course, pro- patibility genes, pin-to-pin and thrum- ticular form of floral heteromorphism motes genetic diversity among the to-thrum pollinations bring pollen is known as distyly, a reference to the seeds and seedlings that constitute the grains into contact with style and long and short styles, but it is impor- next generation, and genetic diversity stigma cells expressing exactly the tant to remember that stamen length within a population is generally con- same proteins. It is the interaction of and, hence, anther position also vary sidered beneficial for the ability of a identical proteins that results in the in distylous flowers. population to adapt to ever-changing abortion of the pollen tube. However, Some two dozen families of flow- environmental conditions. if the genes present in stigmas/styles ering plants have distylous flowers, The essence of distyly is that, al- and pollen are completely different, as and distyly is particularly common in though all pollen and stigmas are func- in pin-to-thrum combinations, no such , the family in which tional, only pin and thrum combina- interaction occurs, the pollen tube func- Mitchella is classified. Other examples tions will succeed and all pin-to-pin tions normally, and this cross-pollina- of plants with distylous flowers in- and all thrum-to-thrum combinations tion results in a fertilization between clude primroses (Primula), flax are incompatible. Clearly, something genetically different gametes. (Linum), and loosestrife (Lythrum). (In beyond mere length of stamens and There are two basic variations in fact, some loosestrife species have styles must be operating to control the the generalized self-incompatibility tristylous flowers, i.e., short, medium, success or failure of pollination in (See Partridgeberry, page 8) Page 4 March 2012 Bulletin of the Virginia Native Plant Society

•Partridgeberry distylous self-incompatibility are pollinations and maintain robust, ge- (Continued from page 4) known, there are just two self-incom- netically diverse, populations. Simple system described above, distinguish- patibility alleles, S and s, and all indi- plants? Hardly! able by the details of pollen genetics. viduals in a population are either Ss or W. John Hayden, VNPS Botany Chair In some cases, it is strictly the genetic ss. Further, the S allele is dominant over Sources constitution of the pollen cells that de- s. Successful pollination (and subse- 1. Ganders, F. R. 1975. Fecundity in termines compatible/incompatible quent fertilization) is possible only in distylous and self-incompatible homo- combinations; such systems are termed Ss X ss crosses; even though such stylous plants of Mitchella repens “gametophytic.” In other cases, called crosses share the s allele from both par- (Rubiaceae). Evolution 29: 186-188. ents, the effect of the S allele dominates 2. Ganders, F. R. 1979. The biology of “sporophytic incompatibility,” it is the heterostyly. New Zealand Journal of so these crosses are effective. As in any genetic constitution of the diploid pol- Botany 17: 607-635. len parent that matters, even though the self-incompatibility system, self-pollina- 3. Hicks, D. J., R. Wyatt, and T. R. Meagher. haploid pollen grains carry just one of tions or crosses involving the same geno- 1985. Reproductive biology of distylous the incompatibility alleles; this is be- types (ss X ss or Ss X Ss) fail. In theory, partridge berry, Mitchella repens. Ameri- cause the pollen grain surface is built because the incompatibility alleles are can Journal of Botany 1503-1514. not just by the haploid cells of the pol- tightly linked to the genes controlling 4. Keegan, C.R., R. H. Voss, and K. S. Bawa. len grain itself, but also by other dip- style and stamen length, distylous 1979. Heterostyly in Mitchella repens loid cells of the anther tapetum, so these sporophytic incompatibility mecha- (Rubiaceae). Rhodora 81: 567-573. pollen grains actually express two in- nisms should result in a nearly 1:1 ratio compatibility alleles. Still, the basic of pin populations to thrum populations. principles of incompatibility apply, Tallies of floral form in natural popula- only unique combinations of alleles tions support the predicted 1:1 ratio, not result in successful pollination events. just for partridge berry, but for other The form of self-incompatibility distylous species as well. found in partridge berry is of the sporo- Next time you stumble upon par- phytic type as described above, but in tridge berry while rambling through the combination with floral heteromor- woods, pause for a moment to ponder phism (distyly), there are a few addi- how these seemingly simple, dainty, tional complications. In all cases for jewel-like plants engage an intricate re- which the underlying genetics for productive system to control compatible