milkweeds

 Central Ohio Prairie Milkweeds
(Asclepias syriaca and A. sullivantii)

Milkweeds at Killdeer

Two milkweed species having a markedly similar growth form and flower morphology co-occur and flower simultaneously at Killdeer Plains Wildlife Area in north-central Ohio. These are Asclepias syriaca (common milkweed, which is indeed common) and A. sullivantii (Sullivant’s milkweeed, an uncommon prairie specialist). 

Sullivant’s milkweeed has large flowers arranged in an upright umbel, and leaves that are sessile and glabrous beneath.

Asclepias sullivantii

Asclepias sullivantii

Common milkweed has moderate-sized flowers arranged in a globose umbel, and petiolate leaves that are pubescent beneath.

Asclepias syriaca

Asclepias syriaca

For several years the Klips lab has been studying various aspects of the reproductive ecology of these milkweeds at Killdeer. This has included documentation of hybridization between these two species. Hybridization is very uncommon in milkweeds, and we are interested in seeing whether hybrids may serve as a bridge to gene flow between the two sympatric species. 

Asclepias sullivantii X syriaca hybrid

Asclepias sullivantii X syriaca hybrid

 The Pollination of Milkweeds

Milkweed pollination involves the transfer of pollen not as individual grains gathered from several flowers, but instead in many-grained masses called “pollinia,” each consisting of all the grains of an entire half-anther. One pollinium is capable of fertilizing all of the ovules in an ovary. The pollinia are in pairs called “pollinaria.” Below is a pollinarium of Sullivant’s milkweed.

ASCLSULL pollinium ruler_

A milkweed pollinarium consists of a pair of pollinia joined at the corpusculum.

Below, see a microscopic view of pollinaria from two milkweed species. Note especially the outer convex surface of each pollinium, from which the pollen tubes emerge.

pollinaria

Pollinaria of two prairie milkweed species.

Milkweed flowers have an unusual morphology related to their exceptional pollination mechanism.  Externally, the center of each flower consists of five colorful reflexed petals concealing five small sepals, and a very conspicuous array of five nectar reservoirs called “hoods,” each of which is over-topped by a radially-oriented “horn” arising from deeper within the hood. In each of the 5 spaces between pairs of hoods lies a stigmatic chamber formed by the fusion of adjacent half-anthers. A knob-like structure termed the “corpusculum” sits at the upper end of each chamber. The corpusculum is a connective between pollinia.

MOUSEOVER IMAGE TO SEE FEATURES LABELLED

Asclepias sullivantii flower

A nectar-foraging insect inadvertently inserts some fast-moving body part –typically a tarsal claw — into a narrow slit in the corpusculum and so withdraws the wishbone-shaped pollinarium. Similar pollinator movements cause pollinium insertion into a stigmatic chamber of a subsequently-visited flower. Below, see honeybees foraging on Sullivant’s milkweed at Killdeer Plains during the summer of 2009.

 

The photos below show flowers (left) without any pollinaria removal or pollinium insertion; (center) pollarium removed, and; (right) pollinium inserted into stigmatic chamber.

asclsullpollin

Milkweed pollination external details.

The center of a milkweed flower is occupied by two separate, superior ovaries topped by a cap-like structure –the gynostegium –composed of five fused stigmas. The side of the gynostegium is in turn surrounded by fused anther-wings, making up the stigmatic grooves and also pockets within which lie the pollinaria.

Below, see a longitudinal section of a milkweed flower with a pollinium situated within a stigmatic chamber.

milkweed longisection

Longitudinal section of pollinated milkweed flower.

The stigmatic surface lies directly across from the outer convex surface of the pollinium, whence emerge the pollen tubes. Below, see the path that pollen tubes would take in their journey to the ovules, all of which are located in the same ovary.

milkweed flower style path

Path of pollen tubes through style.

A microscope view of a gently crushed milkweed ovary, stained with aniline blue and viewed with UV fluorescence shows pollen tubes entering the ovules.

ASCLSULL pollen tubes

Pollen tubes entering ovules, viewed using UV fluorescence microscopy.

One of the two ovaries is connected via stylar tissue to three adjacent stigmatic grooves, while the other is serviced by only two. Accordingly, it is possible for one flower to set two fruits, each sired by a different pollinium. The milkweed fruit is a follicle containing a little over 100 seeds.  Below, see a pair of “twinned” follicles on common millweed. 

Twin common milkweed follicles.

Twin common milkweed follicles.