Evidence of hybridity in invasive watermilfoil (Myriophyllum) populations

Abstract

Invasions of nonindigenous species have caused ecological devastation to natural communities worldwide, yet the biological bases for invasiveness remain poorly understood. Our studies of invasive watermilfoil (Myriophyllum) populations revealed widespread polymorphisms in biparentally inherited nuclear ribosomal DNA sequences, which were not detected in populations of native North American species. Subclones of the polymorphic regions revealed the occurrence of distinct sequences matching those acquired from both nonindigenous and native North American species. Molecular data demonstrate clearly that invasive watermilfoil populations in North America have resulted from hybridization between nonindigenous and native species. These observations suggest that invasiveness in these aggressive aquatic weeds may be linked to heterosis maintained by vegetative propagation.

Fig. 1.

Single most parsimonious tree (317 steps) of Myriophyllum species constructed from sequence data of entire nrITS (CI: 0.808; RI: 0.903). Numbers above branches are bootstrap values; those below are branch lengths. Characters after species names refer to states (U.S.) and accession numbers for collection localities. *, parental species of hybrid taxa.

Fig. 2.

Single most parsimonious tree (72 steps) of a subset of Myriophyllum species illustrating M. heterophyllum × M. pinnatum hybrids (including sequence clones from hybrid populations in bold) constructed from entire nrITS sequence data (CI: 0.972; RI: 0.986). Numbers above branches refer to branch lengths. Characters after species names refer to states (U.S.) and accession numbers for collection localities.

Fig. 3.

Single most parsimonious tree (25 steps) of a subset of Myriophyllum species illustrating evidence of M. spicatum × M. sibiricum hybrids (including sequence clones from hybrid populations in bold) constructed from nrITS-2 sequence data (CI: 1.0; RI: 1.0). Numbers above branches refer to branch lengths. Characters after species names refer to states (U.S.) and accession numbers for collection localities.