Phylogeny of the Order Siphonaptera Based on Molecular Data

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Alison Swindle and Dr. Michael F. Whiting, Zoology

Background

The insect order Siphonaptera (fleas) has been historically significant for thousands of years. Beginning as early as 430 B.C. outbreaks of the bubonic plague have swept across continents and killed millions of people. Before fleas were linked with the plague, they were a little known order. Karl Linne recognized only two species of fleas in his “Systemae Naturae” in 1735. From 1735 to 1897 the described species count increased slowly from two up to sixty-eight. After 1897, and the recognition that fleas may be carriers of the plague, there was a rapid escalation in the research and interest in the order, and the number of species described reached the rate of 250 per decade (Holland, 1964). Currently there are approximately 2380 described species placed in 238 genera, but the further classification into families and suborders remains tentative (Dunnett and Mardon, 1991).

The order Siphonaptera has been an enigma for taxonomists for many years. Fleas are laterally compressed, wingless insects that range from 1 – 10 mm in length. The head is usually small and shield or helmet shaped, and compound eyes are absent. Mouthparts are specialized for piercing and sucking, and the digestive system is designed for blood. These morphological specializations make it possible for fleas to be entirely ectoparasitic, but they also make it difficult to find distinguishing characters on which to base taxonomy above the generic level (Holland, 1964). The highly derived state of flea morphology has resulted in questionable classification within the order Siphonaptera, and a lack of solid support for any group as the sister taxon to the fleas. Based on the similarity of sperm structure (Baccetti, 1972; Gassner et al., 1972; Rothschild, 1965; Smith, 1968), and the recent morphological studies of Kristensen (1981, 1991), a close relationship between Siphonaptera and Mecoptera has been suggested. Recent molecular studies have also suggested this relationship, although taxon sampling was very poor, including only one species of Siphonaptera and one or two species of Mecoptera. A more extensive study by Whiting et al. (1997) supported the Mecoptera + Siphonaptera group, but suggests that Siphonaptera is nested within the Mecoptera. To this point there has been no molecular study that has considered fleas below the ordinal level. The purpose of this study was to collect a large body of molecular data from a wide sampling of both Siphonaptera and Mecoptera in order to investigate the phylogeny within the order Siphonaptera, as well as the relationship of Siphonaptera to Mecoptera.

Methods

All previous attempts at classification of the order Siphonaptera have been based entirely on morphology. The use of DNA sequence data from multiple genes provides independent character sets upon which phylogeny may be based. To obtain molecular data DNA is extracted from the ground tissues of fleas and subjected to Polymerase Chain Reaction (PCR). PCR is a process by which a particular region of DNA can be selectively amplified or duplicated many times. After amplification the desired product is purified and subjected to cycle sequencing, and analyzed by the ABI automated sequencer. The result is an electronic output of the sequence of the desired region of DNA.

Conclusions

Independent and combined analysis was performed on sequences from four genes for 63 different specimens. These data were used as the basis for the reconstruction of the relationships within the order Siphonaptera as well as the relationship of Siphonaptera to other insect orders, specifically Mecoptera. These results support the surprising conclusions reached by Whiting et al. in 1997, specifically that the fleas are nested within the Mecoptera. At the familial level within the order Siphonaptera, the Ceratophyllidae are the only group that is consistently supported. The lack of support for all other families within the order is not unexpected, and confirms the necessity for a thorough study which considers both the molecular as well as the morphological data in an attempt to disperse the confusion that has historically been found in flea taxonomy. The results of this study make it painfully clear that Siphonaptera is a very neglected group within insect systematics, and there is much work to be done before the evolutionary picture of the fleas becomes clear.

Works Cited

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