GENETIC CONSTRAINTS AND THE PHYLOGENY OF INSECT-PLANT ASSOCIATIONS - RESPONSES OF OPHRAELLA-COMMUNA (COLEOPTERA, CHRYSOMELIDAE) TO HOST PLANTS OF ITS CONGENERS

We ask whether patterns of genetic variation in a phytophagous insect' s responses to potential host plants shed light on the phylogenetic hi story of host association. Ophraella communa feeds chiefly, and in eas tern North America exclusively, on Ambrosia (Asteraceae: Ambrosiinae). Using mostly half-sib breeding designs, we screened for genetic varia tion in feeding responses to and larval survival on its own host and o n seven other plants that are hosts (or, on one case, closely related to the host) of other species of Ophraella. We found evidence for gene tic variation in feeding responses to five of the seven test plants, o ther than the natural host. We found no evidence of genetic variation in feeding responses to two plant species, nor in capacity for larval survival on six. These results imply constraints on the availability o f genetic variation; however, little evidence for constraints in the f orm of negative genetic correlations was found. These results are inte rpreted in the context of a provisional phylogeny of, and a history of host shifts within, the genus. Ophraella communa does not present evi dence of genetic variation in its ability to feed and/or survive on So lidago, even though it is probably descended from a lineage that fed o n Solidago or related plants, possibly as recently as 1.9 million year s ago. Genetic variation in performance on this plant may have been lo st. Based on evidence for genetic variation and on mean performance, b y far the greatest potentiality for adaptation to a congener's host wa s evinced in responses to Iva frutescens, which not only is related an d chemically similar to Ambrosia, but also is the host of a closely re lated species of Ophraella that may have been derived from an Ambrosia -associated ancestor. Genetic variation in O. communa's capacity to fe ed and/or survive on its congeners' hosts is less evident for plants t hat do not represent historically realized host shifts (with one excep tion) than for those that may (but see Note Added in Proof). The resul ts offer some support for the hypothesis that the evolution of host sh ifts has been guided in part by constrained genetic variation.