Patterns and sources of variation in pollen deposition and pollen tube formation in flowers of the endemic monoecious shrub Cnidoscolus souzae (Euphorbiaceae)

Abstract

Pollen deposition and pollen tube formation are key components of angiosperm reproduction but intraspecific variation in these has rarely been quantified. Documenting and partitioning (populations, plants and flowers) natural variation in these two aspects of plant reproduction can help uncover spatial mosaics of reproductive success and underlying causes. In this study, we assess variation in pollen deposition and pollen tube formation for the endemic monoecious shrub Cnidoscolus souzae throughout its distribution range in Mexico, and determine how this variation is structured among populations, plants and flowers. We also infer the relative importance of pollen quantity and quality in determining pollination success in this species. While we found no evidence suggesting that pollen receipt limits C. souzae reproduction across 19 populations, we did find extensive variation in pollen load size and pollen tube number per flower. Total variation in pollen receipt and pollen tube number was mostly explained by intra-individual and among-population variance. Furthermore, pollen load size had a stronger effect on the number of pollen tubes at the base of the style than pollen germination rate, suggesting that pollen quantity may be more important than quality for pollen tube success in C. souzae. Our results suggest that both small within-plant flower differences and broad-scale differences in community attributes can play an important role in determining pollination success. We emphasise the need to evaluate patterns and sources of variation in pollen deposition and pollen tube formation as a first step in understanding the causes of variation in pollination success over broad spatial scales.

Floral traits are associated with the quality but not quantity of heterospecific stigmatic pollen loads

“…Associations between floral traits, HP reception or donation and network properties are variable and even contradictory [3,26,27]. In general, stigma size, flower symmetry and floral size increase the likelihood and intensity of HP reception [3,27,28]. Short styles have been suggested to increase or decrease HP susceptibility [1,4,27].…”

Partial and asymmetrical reproductive isolation between two sympatric tropical shrub species: Cnidoscolus aconitifolius and C. souzae (Euphorbiaceae)

“…Given that the study species are closely related, this high phylogenetic relatedness may at least partially explain why pollen-pistil incompatibility was the weakest pre-zygotic barrier. Moreover, similarities between the study species in terms of flower morphology and floral biology (Arceo-Gómez et al, 2016;Munguía-Rosas & Jácome-Flores, 2020) could explain why other pre-pollination, pre-zygotic barriers, such as pollinator differentiation (RI: 0.38-0.50), were also relatively weak in this pair of Cnidoscolus species. Thus, our findings have added to the known exceptions (e.g., Costa et al, 2007;Jewell et al, 2012;Johnson et al, 2015;Liao et al, 2019) to the previously depicted pattern regarding the greater strength exhibited by pre-zygotic barriers (Baack et al, 2015;Lowry et al, 2008).…”

“…Cnidoscolus souzae is a narrow endemic species, and its entire coarse-grained distribution range is fully embedded within the wider distribution range of C. aconitifolius (Arceo-Gómez et al, 2016;Ross-Ibarra & Molina-Cruz, 2002; Figure 1a).…”

“…Although allopatric populations of C. aconitifolius exist, we experimentally assessed reproductive isolation in sympatry only given that, under this condition, the actual RIBs are far more detectable and play a more important role in preventing gene flow (Coyne & Orr, 2004). A comparison of two independent studies with C. aconitifolius (Munguía-Rosas & Jácome-Flores, 2020) and C. souzae (Arceo-Gómez et al, 2016) revealed that the flower morphology of these two species is very similar. The flowering phenologies overlap to some extent, and they share their main flower visitors (bees and butterflies).…”

Temporal patterns of pollen shedding for longleaf pine (Pinus palustris) at the Escambia Experimental Forest in Alabama, USA

“…Taylor (1961) discovered an empirical law in ecology, which links the variance of the population for a species in a habitat area to its corresponding mean by a power-law relationship. This scaling relationship has been expanded from population density and widely observed in ecology (e.g., Cohen et al, 2013;Taylor, 2019), such as pollination success in a shrub species at Mexico's Yucatan Peninsula (Arceo-Gómez et al, 2016). Our recent studies indicated that some plants followed Taylor's law in their growth traits (Chen et al, 2017;Chen, 2020;Chen & Chen, 2020), but it was not tested in pollen shedding.…”