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).