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). Here we introduce the MRCALink (most-recent-common-ancestor link) method and use it to investigate the possibility of grass-epichloё 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 Ptree 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 ParaFit or the Mantel test by eliminating an extreme (but previously unrecognized) distortion of node-pair sampling. Analysis of 26 grass species-epichloё 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.

Host-parasite associations are born by cospeciations or by host switches. Feather lice have traditionally been used as model parasites for studying these events and underlying ecological factors. By now tens of analyses have addressed comparisons of host and parasite phylogenies to study cospeciations and host switches in lice, however, these analyses are strongly biased towards the temperate zone. Tropical environment could provide new insight into the origin of host-parasite interactions, because it is ecologically unique. This work aims to supplement the knowledge of host-parasite associations in lice using coevolution analyses of two feather lice genera and their passerine hosts in tropical rainforest in Cameroon. It shows that lice in the tropics cospeciate rarely. To assess whether host switches are non-random and occur preferentially between hosts with specific traits, this work also analyses relations between parasite genetic distances and hosts' trait similarities. No effect of host morphology and spatial distribution was found. However, genetic distances of the lice strongly correlate with genetic distances of their hosts.