The animal family tree may not be filled just with forks, but may also contain knots: hybrid species with two different ancestors rather than one, according to a team of Penn State researchers.
"We are looking for the origin of species," says Dr. Dietmar Schwarz, post-doctoral researcher in entomology. "In animals, people envision the formation of a new species by a split of one ancestral species to two derived species or a branching of one species from another."
However, according to Schwarz, another way to get a new species is for two species to hybridize – mate with each other – forming a new species lineage while the parental species persists.
"It is thought that over 50 percent of plants came into being this way, but that this mechanism played hardly any role in animals," says Schwarz. "Hybridization was seen as an accident resulting in sterile offspring like mules, but not as the beginning of a new species."
In the plant world, many plants create hybrid species by doubling the number of chromosomes in the parent for the offspring generation, but some hybrids, like the sunflower species, do contain the same number of chromosomes as the parents. For a handful of animal species – some fish – genetic information suggests hybridization as the most likely form of origin, but only a little is known about the actual mechanism.
"The real problem for the formation of a new species is ecological compatibility," says Schwarz. "Is there some ecological niche that is not already in use?"
Schwarz; Bruce A. McPheron, professor of entomology and associate dean for research and director of Pennsylvania Agricultural Experiment Station; and Benjamin M. Matta and Nicole Shakir-Botteri, Penn State undergraduate biology majors who have since graduated, present their findings on a hybrid tephritid fruit fly – the Lonicera fly, a member of the Rhagoletis pomonella species complex -- and its host resource, the non-native brushy honeysuckle, in the July 28 issue of the journal Nature.
Exotic brushy honeysuckle may have come to North America as early as 1750, but probably became established in 1880 when the U.S. Department of Agriculture introduced it as an ornamental and wildlife plant. The plant now grows wild throughout the Northeastern United States. The introduction of this plant, however, provided an unused resource that could initiate a host shift for the flies. Rather than gradually adapt to a new host plant, the flies hybridized.
Tephritid fruit flies are specialized fruit parasites. Each species of fly uses only one berry-bearing plant as its host where adult flies mate on the bushes and lay their eggs in the berries in the summer. The larvae hatch and grow in the fruit, destroying the berry. When mature, the larvae crawl out of the berry, drop to the ground and pupate, waiting through the winter to emerge as adults. This type of host-dependent lifestyle is very common. More than 50 percent of all animal species, many of them tiny plant eaters and parasites, are believed to be host specialists like Rhagoletis tephritid flies.
Blueberry tephritid flies mate on blueberry bushes and snowberry tephritid flies mate on snowberry bushes. This helps to avoid matings with a partner from the wrong host plant and keeps the two different parent species separate. Matings between species occur on occasion, but the hybrid offspring are short lived because they are believed to do much worse on either blueberry or snowberry than its parents.
Schwarz thinks that the introduction of invasive, brushy honeysuckle caused a breakdown of this isolation and created a fly that completes its reproductive cycle in the brushy honeysuckle berry.
"As far as we can tell, we think the new species is already reproductively isolated," says Schwarz. "They seem to be in a niche on the brushy honeysuckle where the parent species cannot compete."
Reproductive isolation – not interbreeding with either of the parent species -- is one requirement for a species.
The researchers note that it is very difficult to detect new species like the brushy honeysuckle fly. The two parent species look like identical twins and can only be distinguished by their different genetic makeup. It is also difficult to determine that the fly is a true hybrid species and not a pre-existing, but undiscovered, species that was formed without hybridization. The researchers can estimate what the first generation of a cross between blueberry and snowberry flies looks like genetically, but by the time the species is established, there are no longer any offspring of the original cross mating around to investigate.
So, Schwarz used a standard approach and looked at genetic markers in the brushy honeysuckle fly. He compared those markers with genes from other tephritid flies. He found that the blueberry and snowberry flies, because of their specific genetic makeups, were the only possible combination of parents for the brushy honeysuckle fly.
"This kind of comparison can only be done because so much is known about this type of fly," says Schwarz. "But there might be a lot more animal hybrid species than previously thought if researchers take a closer look."
Human activity has resulted in the rapid mixing of plants and animals that had no previous contact with each other. According to the researchers, such man-made disturbances could create many more situations in which new hybrid species could form.
The National Science Foundation supported this research.