Adaptive responses
Evaluation of the potential adaptive responses to within-species
variation in HP receipt across the landscape (Arceo-Gómez & Ashman
2014a, Arceo-Gómez et al. 2016a, Moreira-Hernandez & Muchhala 2019)
remains a promising field of study. Here, several avenues of research
exist. First, studies that evaluate the potential for plant populations
to adapt to different HP transfer regimes (e.g. Hopkins & Rausher 2012,
Arceo-Gómez et al. 2016a). This can be achieved by evaluating the
potential for natural selection on traits associated with HP tolerance
or avoidance strategies under different HP transfer environments (e.g.
Hopkins & Rausher 2012, Tong & Huang 2016), and/or via reciprocal
transplant experiments that evaluate patterns of local adaptation
(Arceo-Gómez & Ashman 2014a). Hand-pollination studies that evaluate
population-level variation in HP effects under controlled conditions
would also be valuable to elucidate the potential for the evolution of
HP tolerance strategies in nature (e.g. Arceo-Gómez et al. 2016a, Tong
& Huang 2016). Furthermore, few studies have measured traits and
fitness in communities of varying species composition (Johnson &
Stinchcombe 2007), thereby assessing the potential role of diffuse
selection on species evolutionary trajectory as a response to HP
receipt.
Second, it is also important to design these studies in a way that we
can separate adaptive responses from the male (pollen) and female
(style/stigma) perspectives in order to fully assess the adaptive
potential of plants to HP effects. Such studies would also help to
pinpoint the exact mechanisms mediating HP tolerance and avoidance. For
instance, although several mechanisms/traits conferring HP tolerance
have been proposed such as longer styles or dry stigmas (reviewed in
Ashman & Arceo-Gómez 2013), to date very few studies have attempted to
test these predictions (Tong & Huang 2016, Arceo-Gómez et al. 2019b).
Thus, our understanding of the potential traits and mechanisms
conferring HP tolerance is still very limited. Third, there is also
evidence indicating that HP receipt may play an important role in mating
system evolution and in altering the genetic architecture of plant
populations, with so far unknown consequences (Arceo-Gómez & Ashman
2014b). For instance, higher levels of outcrossing as a result of
greater HP receipt (Arceo-Gómez & Ashman 2014b), could increase genetic
diversity and the rate of evolutionary change within populations (Hughes
et al. 2008). An increase in genetic diversity could also help generate
and maintain species diversity at the community-level via effects on
population-level fitness (Vellend & Geber 2005, Hughes et al. 2008). In
spite of these tantalizing possibilities, to my knowledge, this very
promising avenue of research remains unexplored. Thus, studies that link
within-species variation in patterns of HP receipt, outcrossing rates
and levels of genetic diversity across populations with patterns of
species diversity across communities could offer transforming insights
on the role of HP receipt in shaping patterns of diversity not only
across spatial, but across biological scales (from genes to
communities).