HOST-PLANT SWITCHES AND THE EVOLUTION OF CHEMICAL DEFENSE AND LIFE-HISTORY IN THE LEAF BEETLE GENUS OREINA

Insect-plant interactions have played a prominent role in investigatin g phylogenetic constraints in the evolution of ecological traits. The patterns of host association among specialized insects have often been described as highly conservative, yet not all specialized herbivorous insect lineages display the same degree of fidelity to their host pla nts. In this paper, we present an estimate of the evolutionary history of the leaf beetle genus Oreina. This genus displays an amazing flexi bility in several aspects of its ecology and life history: (1) host pl ant switches in Oreina occurred between plant families or distantly re lated tribes within families and thereby to more distantly related pla nts than in several model systems that have contributed to the idea of parallel cladogenesis; (2) all species of the genus are chemically de fended, but within the genus a transition between autogenous productio n of defensive toxins and sequestration of secondary plant compounds h as occurred; and (3) reproductive strategies in the genus range from o viparity to viviparity including all intermediates that could allow th e gradual evolution of viviparity. Cladistic analysis of 18 allozyme l oci found two most parsimonious trees that differ only in the branchin g of one species. According to this phylogeny estimate, Oreina species were originally associated with Asteraceae, with an inclusion of Apia ceae in the diet of one oligophagous species and an independent switch to Apiaceae in a derived clade. The original mode of defense appears to be the autogenous production of cardenolides as previously postulat ed; the additional sequestration of pyrrolizidine alkaloids could have either originated at the base of the genus or have arisen three times independently in all species that switched to plants containing these compounds. Viviparity apparently evolved twice in the genus, once wit hout matrotrophy, through a retention of the eggs inside the female's oviducts, and once in combination with matrotrophy. We hypothesize tha t the combination of autogenous defense and a life history that involv es mobile externally feeding larvae allowed these beetles to switch ho st plants more readily than has been reported for highly conservative systems.