What Governs Protein Content of Pollen: Pollinator Preferences, Pollen-Pistil Interactions, or Phylogeny?

Abstract

Pollen ranges from 2.5% to 61% protein content. Most pollen proteins are likely to be enzymes that function during pollen tube growth and subsequent fertilization, but the vast range of protein quantity may not reflect only pollen-pistil interactions. Because numerous vertebrate and invertebrate floral visitors consume pollen for protein, protein content may influence floral host choice. Additionally, many floral visitors pollinate their host plants. If protein content influences pollinator visitation, then pollinators are hypothesized to select for increased protein content of host plants. We analyzed or gleaned from the literature crude pollen protein concentrations of 377 plant species from 93 plant families. Using this database, we compared pollen protein concentration with (1) pollination mode, (2) pollen collection by bees, and (3) distance from stigma to ovule, after accounting for phylogeny through paired phylogenetic comparisons and a nested ANOVA including taxonomic rank. We found that pollen protein concentrations were highly conserved within plant genera, families, and divisions. We found that bees did not collect pollen that was unusually rich in protein, whether they pollinated or merely robbed their host plant. Plant species with vibratile pollination systems, which require visitation by pollen-collecting bees in order to transfer pollen, tended to have very protein-rich pollen, but it was not clear whether this was due to plant enhancement of pollinator rewards or to the possession of very small pollen grains. We found that zoophilous species were not statistically richer in pollen protein than anemophilous species after accounting for phylogeny, although the three most species-rich anemophilous clades surveyed were generally poor in protein. Plant genera hosting specialist pollen-collecting bees did not have particularly protein-rich pollen. Both mass of protein per pollen grain and pollen grain volume were correlated with stigmaovule distance. We suggest that the need for growing pollen tubes probably plays a more important role in determining pollen protein content than rewarding pollinators.

Macronutrient ratios in pollen shape bumble bee ( Bombus impatiens ) foraging strategies and floral preferences

“…To test if foraging preferences among host-plant species were associated with pollen nutritional quality, we collected extensive simultaneous foraging visitation data for B. impatiens colonies to multiple plant species (controlling for phenology and floral area/resource availability) and analyzed the nutritional content (carbohydrate, protein, and lipid concentrations) of each host-plant species' pollen ( Table 1). All nutritional values were within the expected range for pollen (18). Foraging data and methodology were previously published in ref.…”

“…However, pollen nutritional quality varies widely among plant species, ranging from 2-60% protein and 1-20% lipids by weight (10,18); thus, it likely is critical that bees selectively collect pollen species with the necessary nutritional quality to support their needs (1). The protein and amino acid concentrations of pollen modulate immunocompetence in honey bees (16,19) and reproduction (ovary activation and larval development) in bumble bees (12,14,(20)(21)(22).…”

The Effects of Pollen Protein Content on Colony Development of the Bumblebee, Bombus Terrestris L.

“…Pollen may also be affected by Effects of pollen protein content on Effects of pollen protein content on B. terrestris B. terrestris factors such as drying, ageing, or storage conditions (Ribeiro et al, 1996). The protein content of pollen can also vary widely, ranging from 2.5 to 61% and is generally accepted as a reliable measure of its nutritional value (Roulston et al, 2000). Similarly, the crude protein content of pollen collected by the honey bee typically ranges from 7.5 to 35% (Schmidt and Buchmann, 2003).…”

Morphological characterization of pollens from three Apiaceae species and their ingestion by twelve-spotted lady beetle (Coleoptera: Coccinellidae)

“…Although it is not possible to affirm, the lower ingestion of Apiaceae pollens might be due to the higher protein content. Roulston et al (2000) recorded that the protein content of the Z. mays pollen is 23.9%, and 29.0% for the Anthriscus sylvestris (L.) (Apiacae). Nevertheless, data in the literature as to the protein content of Apiaceae pollens evaluated in the present study were not found.…”

“…The ingestion of corn pollen grains (Zea mays L., Poaceae) by larvae and adult C. maculata has been recorded, and whose mean average protein content was estimated at 24% (Roulston et al, 2000), making up quite an attractive botanical species for this coccinellid (Smith, 1965;Lundgren et al, 2005;Weber and Lundgren, 2011). Hippodamia convergens Guérin-Méneville, 1842 (Coleoptera: Coccinelidae) adults collected in organic vegetable crops were also determined as being able to ingest pollen grains from ten botanical families, including Apiaceae, although their species were not identified (Medeiros et al, 2010).…”