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).