Pollen-Pistil Interactions and Their Role in Mate Selection.

As a group, the flowering plants have remarkably diverse modes of reproduction that run the gamut from clonal propagation to obligate outcrossing. Articles in this volume focus on sexual reproduction, and there is great diversity even within this slice of the reproduction spectrum. Like all diverse systems, reproductive characters are the end results of molecular and genetic mechanisms that generate diversity. Selective pressures that act upon this diversity lead to the differentiation of lineages and themaintenance of their identity. The seemingly minuscule slice of the angiosperm life cycle represented in the postpollination to prezygotic phase, which is dominated by pollen-pistil interactions, plays a large role in these evolutionary processes. The pistil is unique to angiosperms. It serves a protective role and functions as a conduit for pollen tubes to grow to the ovary, but it also provides a venue for pollen-pistil interactions that regulate pollen tube growth and, hence, fertilization. Although the earliest diverging angiosperm lineages do not possess the full set of canonical angiosperm pistil traits (Endress and Doyle, 2009; Endress, 2011, 2015; Lora et al., 2016), the core lineages (which display the most diversity) have a stigma specialized for the receipt of pollen and a style that pollen tubes must traverse before fertilization can occur. Some authors attribute angiosperm diversity to phenomena that depend on pollen-pistil interactions taking place in stigmas and styles, such as pollen competition and self-incompatibility (SI; Mulcahy and Mulcahy, 1988; Williams, 2012). Therefore, the evolved state of the pistil, as the organ that stands between pollen receipt and the ovule, allows it to assume a gatekeeper function, facilitating the growth of desirable pollen tubes and discouraging invaders and less desirable pollen (Heslop-Harrison, 2000). Our focus is on pollen-pistil interactions that contribute to mate selection in the domains of SI and interspecific pollen rejection. Together, these processes define a window of genetic relationships for successful mating: SI prevents mating between very closely related individuals of the same species, and interspecific pollen rejection prevents crosses that are too distant. SI clearly contributes to a species’ long-term success, as SI lineages are genetically diverse and very persistent (Goldberg et al., 2010; Goldberg and Igi c, 2012). However, transitions from SI to self-compatibility (SC) also are important. SC lineages emerge frequently and may