Mayden and Roe Based on the fossil record, the greatest diversity of jawless vertebrates (agnathans) occurred during the upper Silurian and Early and Middle Devonian geological periods, approximately 420 to 380 million years ago. The majority of agnathans became extinct by the end of the Devonian (360 million years ago) with one or more lineages surviving to give rise to modern jawless vertebrates. Lampreys represent one of the two surviving groups of agnathans (hagfishes are the other group). Living lampreys are classified in the order Petromyzontiformes, which includes 40 extant species. The geographic distribution of lampreys is notable in that they are found in both Northern and Southern hemispheres, but are almost entirely absent from tropical regions. It has been hypothesized that this distribution of lampreys is the result of the breakup of the supercontinent Pangea and the post-Pangean continents of Laurasia and Gondwana. In addition to their interesting geographic distribution lampreys are also remarkable for several reasons: 1) they represent a transitional stage between invertebrate chordates and vertebrates that possess jaws, and as such allow the testing of hypotheses concerning the evolution of characters considered to be unique to vertebrates; 2) they possess a complex life history that involves metamorphosis from a sedentary, filter feeding larval form termed an ammocete, to a sexually reproducing adult; and 3) they can be divided into two groups based on whether the species are parasitic or non-parasitic. Parasitic lampreys include both anadromous species and those restricted to streams and rivers. Species with non-parasitic life-cycles are similar to those with a parasitic life-cycle except that the adults do not feed. Historically, morphological similarities between parasitic and non-parasitic species have been interpreted as evidence that extant non-parasitic species evolved independently on several occasions from different parasitic species. These hypothesized sibling species have been termed "paired species" or "satellite species." Despite agreement among lamprey researchers as to the validity of the satellite species hypothesis, no explicit phylogenetic analysis has been conducted to support such relationships. Drs. Richard Mayden and Kevin Roe at the University of Alabama are seeking to test these hypothesized relationships, and thereby test one of the most remarkable cases of multiple convergence ever suspected in vertebrates. The study will produce phylogenetic hypotheses for all lamprey species using both morphological and molecular (DNA sequences) data sets. Specific objectives are to test hypotheses concerning (1) the evolutionary origins of the parasitic and non-parasitic life histories, (2) the relationships between the various species and genera and families within Petromyzontiformes, (3) the biogeographic history of Petromyzontiformes with reference to the history of the earth and other taxa having similar distributions, and (4) the proposed relationship between lampreys and jawed vertebrates. The project is international in scope, and includes the collection of specimens on several continents (North America, South America, Europe and Australia), and involves collaboration with three acknowledged experts in lamprey biology and systematics: Drs. I. Potter and H. Gill of Murdoch University, Perth, Australia and Dr. C. Renaud from the Canadian Museum of Nature, Ottawa, Ontario, who have primary responsibility for the collection of additional specimens (important for morphological characters and molecular analyses) and the development of morphological data sets. These data will be combined with the molecular data sets based upon the complete DNA sequences of several mitochondrial genes.
