Frugivory: A Retained Characteristic of Microbiotheria?
The mutualistic relationships between Dromiciops and several endemic plants can be traced back in time to their ancestors. These facts furnish fascinating ideas about the trophic habits of past Microbiotheriids and their eco-evolutionary relationships with the temperate flora of southern South America, which may have coevolved during millions of years. Highly specific and asymmetric (i.e., uneven dependence between the plant and the animal) interactions have appeared, such as the seed dispersal relationships between Dromiciops and the hemiparasitic mistletoe Tristerix corymbosus (Aizen, 2003; Amico & Aizen, 2000).
Recent work suggests an inter-dependence between Dromiciops andTristerix , which may also reflect an ancient association between microbiotheriids and mistletoes. Successive phylogenetic reconstructions have pushed the origin of mistletoes back further in time (Liu et al., 2018; Nickrent, Malécot, Vidal-Russell, & Der, 2010), with the growth habit now estimated to have transitioned from root parasite to aerial parasite in the Loranthaceae during the early Eocene (approximately 50 million years ago). Based on fossil reconstructions and the modern-day distribution of lineages on either side of this transition, it is estimated that the shift from understorey up to the canopy occurred in western Gondwanaland. This time is 20 to 30 million years prior to the origin of the modern frugivorous birds, the main dispersers of Loranthaceae mistletoes today (Liu et al., 2018). This temporal mismatch between the origins of mistletoes and their seed dispersers has been previously noted, as early Microbiotheriids are invoked as the most probable agents of mistletoe dispersal prior to the diversification of songbirds (Amico & Aizen, 2000; Restrepo, Sargent, Levey, & Watson, 2002). Recently, this idea has been taken one step further by Watson (2020), who suggested Microbiotheriids may have been indeed the selective agents responsible for the transition within Loranthaceae from root-parasitic shrubs to stem-parasitic mistletoes. By consuming fruits from understorey shrubs and dispersing them up to the canopy,Dromiciops ancestors catalysed the switch in growth habit to the upper canopy, where transitional forms likely parasitised the roots of vascular epiphytes like the present-day Gaiadendron (Restrepo et al., 2002).