  Significant phylogenetic codivergence between plant or animal hosts ($H$) and
their symbionts or parasites ($P$) indicate the importance of their
interactions on evolutionary time scales. However, valid and realistic methods
to test for codivergence are not fully developed. One of the systems where
possible codivergence has been of interest involves the large subfamily of
temperate grasses (Pooideae) and their endophytic fungi (epichloae). These
widespread symbioses often help protect host plants from herbivory and
stresses, and affect species diversity and food web structures. Here we
introduce the MRCALink (most-recent-common-ancestor link) method and use it to
investigate the possibility of grass-epichlo\"e codivergence. MRCALink applied
to ultrametric $H$ and $P$ trees identifies all corresponding nodes for
pairwise comparisons of MRCA ages. The result is compared to the space of
random $H$ and $P$ tree pairs estimated by a Monte Carlo method. Compared to
tree reconciliation the method is less dependent on tree topologies (which
often can be misleading), and it crucially improves on phylogeny-independent
methods such as {\tt ParaFit} or the Mantel test by eliminating an extreme (but
previously unrecognized) distortion of node-pair sampling. Analysis of 26 grass
species-epichlo\"e species symbioses did not reject random association of $H$
and $P$ MRCA ages. However, when five obvious host jumps were removed the
analysis significantly rejected random association and supported
grass-endophyte codivergence. Interestingly, early cladogenesis events in the
Pooideae corresponded to early cladogenesis events in epichloae, suggesting
concomitant origins of this grass subfamily and its remarkable group of
symbionts. We also applied our method to the well-known gopher-louse data set.
