"Nobody but me will ever grow these in their backyard, notes Joel McNeal, a graduate student in biology at Penn State. He is referring to a kind of Dodder, in this case, Cuscuta rostrata, a parasitic plant that sprouts orangey yellow vines with small bunches of white flowers resembling popcorn.
Joel McNeal
Not all Morning Glories are the friendly type.
Dodders are also known as Beggarweed, Hellweed, Strangle Vine, Love Tangle, Scaldweed, Devil's Guts, and Devil's Hair. A close relative of the Morning Glory—that fast-growing annual vine with the four- to five-inch blooms in pink, white, blue, and red—the Dodder in its many varieties is responsible for millions of dollars of crop damage. Commonly attacked crops include alfalfa, clover, onions, and citrus trees. To combat this parasite, farmers in the United States rely on strong herbicides. Where expensive chemicals are not an option, farmers burn infected crops.
Not surprisingly, the only backyard in which McNeal is growing this imported species is a quarantined greenhouse. Every time he leaves it, he must thoroughly check himself to catch any seeds trying to escape to the rolling hills of Pennsylvania.
Instead of through their roots or green, photosynthetic leaves, Dodders get water and nutrients through sucker-like attachments called haustoria, with which they latch on to their host plants. Basically, it's a Morning Glory that decided to eat other plants, McNeal says with a smile. Most species of Dodder have brightly colored vines and tiny flowers. The seeds start sprouting after the frost, and the vines become apparent in late June or July, explains McNeal. The plant starts out slowly in the cooler temperature but as the weather warms, growth begins to speed up and it turns into a great big mess. The vines can grow to be a half-mile long, tangled up into a colorful, strangling mass of spaghetti or silly string which can cover a couple of square yards. The typical Dodder produces about 1,000 flowers, with four seeds per flower. Each seed—bigger than a grain of sand but smaller than a pinhead—has a hard coat and can lie dormant for 60 years.
The scientific community is not in agreement on the number of Cuscuta species. Estimates vary from 100 to 200, says McNeal. Some are very hard to tell apart. To help differentiate among them, McNeal has created a phylogenetic tree of the Cuscuta genus showing how the species are related to each other. To produce this tree, McNeal collected a variety of Dodders from herbaria, from the wild in different states he has visited, and received seeds from colleagues overseas. McNeal has a USDA permit allowing him to possess the non-native species. The seeds are seen as agricultural pests, he explains. It's illegal to import them without a permit. We don't want the citrus trees in Florida or California to be infected.
McNeal selected specific genes to examine in each plant and amplified them using the Polymerase Chain Reaction (PCR) technique, creating numerous copies of the plant's DNA. Then he sequenced the genes in an automated DNA-sequencing machine. A computer program compares the gene sequences to create the phylogenetic tree. The length of each branch on the tree can be drawn proportional to how many changes have occurred in the DNA sequences of the genes checked. Various programs can also give a confidence rating for each branching point on the tree.
McNeal has also compared the DNA sequences of several Cuscuta species to those of the tobacco plant. While Morning Glories are the closest relative, tobacco is also relatively close, he explains. Tobacco is also one of the first plants to have had its chloroplast genome sequenced, making it a good model for comparison. Chloroplasts not only make a plant's leaves green, they allow the plant to photosynthesize and manufacture its own food from sunlight. McNeal found that while the DNA sequences of some Cuscuta species correspond almost exactly to those of the tobacco plant's chloroplast genome, others are missing large chunks—probably the more missing DNA pieces, the less able the plant is to photosynthesize. McNeal hopes to find out what specific genes were lost when this plant evolved from a garden flower into a virulent parasite.
Joel McNeal is a graduate student in biology; jrm24@psu.edu. He won third prize for Health and Life Sciences in the 2002 Graduate Exhibition. His adviser is Claude DePamphilis, Ph.D., associate professor of biology in the Eberly College of Science, 208 Mueller Lab, University Park, PA 16802; 814-863-6412; cwd3@psu.edu.