Bats As Pollinators – An Introduction to Chiropterophily

Most plants that rely on animals to assist in pollination look to insects. In general, insects are abundant, easy to please, and efficient at transferring pollen. Because insect pollination is such a common scenario, it is easy to overlook pollination that is carried out by vertebrates. Birds are the most prominent pollinator among vertebrates, but mammals participate, too. The most common mammal pollinator is the bat.

About a fifth of all mammal species on the planet are bats, with species estimates numbering in the 1200-1300 range. Bats are the only mammals that can truly fly. They are not blind, nor are they flying rodents, and they are not going to suck your blood (except in very rare cases!). Most bats eat insects, but a small, significant group of them are nectarivorous. Their main food source is the nectar produced within flowers. In the process of feeding, these bats pollinate plants.

Out of 18 families in the order Chiroptera, only two include species with morphologies that set them apart as nectar-feeders. The family Pteropodidae, known commonly as Old World fruit bats or flying foxes, occurs in tropical and subtropical regions of Africa, Asia, Australia, Papa New Guinea, and the Pacific Islands. The family Phyllostomidae, known commonly as American leaf-nosed bats, occurs in tropical and subtropical regions of the Americas. For simplicity’s sake, the former are referred to as Old World bats, and the latter as New World bats. While both groups are similar in that they consist of species that feed on nectar, they are only distantly related, and thus the nectar feeding species in these families have distinct behavioral and morphological differences.

Grey headed flying fox photo credit: wikimedia commons

Grey headed flying fox (Pteropus poliocephalus), a floral visiting bat from Australia (photo credit: wikimedia commons)

More than 500 species of plants, spanning 67 plant families, are pollinated by bats. This pollination syndrome is known as chiropterophily. In general, flowers that use this approach tend to be white or dull in color, open at night, rich with nectar, and musty or rotten smelling. They are generally tubular, cup shaped, or otherwise radially symmetrical and are often suspended atop tall stalks or prominently located on branches or trunks. In a review published in Annals of Botany, Theodore Fleming, et al. state “flower placement away from foliage and nocturnal anthesis [blooming] are the unifying features of the bat pollination syndrome,” while all other characteristics are highly variable among species. The family Fabaceae contains the highest number of bat-pollinated genera. Cactaceae, Malvaceae, and Bignoniaceae follow closely behind.

The characteristics of bat pollinated flowers vary widely partly because the bats that visit them are so diverse. Between the two bat families there are similarities in their nectar-feeding species, including an elongated rostrum, teeth that are smaller in number and size, and a long tongue with hair-like projections on the tip. Apart from that, New World bats are much smaller than Old World bats, and their rostrums and tongues are much longer relative to the size of their bodies. New World bats have the ability to hover in front of flowers, while Old World bats land on flowers to feed. Old World bats do not have the ability to use echolocation to spot flowers, while New World bats do. Fleming, et al. conclude, “because of these differences, we might expect plants visited by specialized nectar-feeding [New World bats] to produce smaller flowers with smaller nectar volumes per flower than those visited by their [Old World bat] counterparts.”

Pollination by bats is a relatively new phenomenon, evolutionarily speaking. Flowers that are currently pollinated by bats most likely evolved from flowers that were once pollinated by insects. Some may have evolved from flowers that were previously bird pollinated. The question is, why adopt this strategy? Flowers that are bat pollinated are “expensive” to make. They are typically much bigger than insect pollinated flowers, and they contain large amounts of pollen and abundant, nutrient-rich nectar. Due to resource constraints, many plants are restricted from developing such flowers, but those that do may find themselves at an advantage with bats as their pollinator. For one, hairy bat bodies collect profuse numbers of pollen grains, which are widely distributed as they visit numerous flowers throughout the night. In this way, bats can be excellent outcrossers. They also travel long distances, which means they can move pollen from one population of plants to an otherwise isolated neighboring population. This serves to maintain healthy genetic diversity among populations, something that is increasingly important as plant populations become fragmented due to human activity.

Pollinating bats are also economically important to humans, as several plants that are harvested for their fruits, fibers, or timber rely on bats for pollination. For example, bat pollinated Eucalyptus species are felled for timber in Australia, and the fruits of Durio zibethinus in Southeast Asia form after flowers are first pollinated by bats. Also, the wild relatives of bananas (Musa spp.) are bat pollinated, as is the plant used for making tequila (Agave tequilana).

Durio sp. (photo credit: wikimedia commons)

The flowers of durian (Durio sp.), trees native to Southeast Asia, are pollinated by bats (photo credit: wikimedia commons)

There is still much to learn about nectarivorous bats and the flowers they visit. It is clear that hundreds of species are using bats to move their pollen, but the process of adopting this strategy and the advantages of doing so remain ripe for discovery. Each bat-plant relationship has its own story to tell. For now, here is a fun video about the bat that pollinates Agave tequilana:


Improving Perennial Crops with Genomics

This is the twelfth in a series of posts reviewing the 17 articles found in the October 2014 Special Issue of American Journal of Botany, Speaking of Food: Connecting Basic and Applied Science.

Genomics: A Potential Panacea for the Perennial Problem by Kendra A. McClure, Jason Sawler, Kyle M. Gardner, Daniel Money, and Sean Myles

Compared to annuals, a small but significant portion of our food comes from perennial crop plants. “Approximately one eighth of the world’s total food-producing surface area is dedicated to perennials,” and while that may seem relatively small, there is a good chance that some of your favorite things to eat or drink are perennial crops (apples, bananas, coffee, citrus, sugar cane, coconut, avocados, olives, grapes, cherries, almonds…just to name a few). However, making improvements to and introducing new cultivars of perennial crops is considerably more challenging compared to annual crops simply due to the nature of perennials. This puts perennial crops at greater risk to threats like pests and diseases, climate change, soil degradation, and water and land shortages. Advances in genomics, “the collection and use of DNA sequence information,” could change this.

Because breeding efforts to improve perennial crops is so challenging, “only a small number of elite varieties become popular, and the amount of genetic diversity represented by commercially successful cultivars is therefore often low.” This suggests that there is incredible potential for improvement in these crops, as long as major hurdles can be overcome. Following is a list of some of those hurdles:

  • Time – Most perennial crops have “extended juvenile phases,” meaning they won’t produce fruit for as much as ten years, considerably delaying evaluation of the final product.
  • Space – Perennial crops, especially trees, are large compared to annual crops, so the area required for evaluation is extensive.
  • Infrastructure – “Many perennials require trellis systems, extensive land preparation, and substantial costs for specialized equipment and skilled horticultural labor.”
  • Complex Evaluations – Automated assessments are “either unavailable or poorly developed,” so evaluations that include “size, shape, color, firmness, texture, aroma, sugars, tannins, and acidity” require “tasting panels” to ensure that the final product “satisfies consumer demands.” This process is expensive, and it differs depending on whether the crop will be consumed fresh or processed.
  • Vegetative Propagation – “Many perennials suffer from severe inbreeding depression when selfed,” so cultivars are maintained through vegetative propagation. This is a plus, because it means that the fruits of perennial crops are reliably uniform, so growers and consumers know what to expect year after year. However, this also means that while pests and pathogens evolve, the crops do not, making them more susceptible to such threats. Additionally, the “long histories” of certain cultivars “discourages [growers] from undergoing the risk of trying recently developed cultivars.”
  • Consumer Preferences – “Consumers often exhibit an irrational reverence for ancient or heirloom varieties,” despite the fact that the development of new varieties can result in crops that are higher yielding, resistant to pests and diseases, tastier, more nutritious, more suitable for storage, and require fewer chemical inputs. This obsession with traditional varieties leaves a “tremendous amount of untapped genetic potential for the improvement of perennial crops.”
"Modern avocado breeding still depends heavily on open-pollination because of the difficulty associated with making controlled crosses." (photo credit: wikimedia commons)

“Modern avocado breeding still depends heavily on open-pollination because of the difficulty associated with making controlled crosses.” (photo credit: wikimedia commons)

Apart from issues of social and cultural preference, the challenge of breeding perennial crops comes down to time and money. Advances in genomics can help offset both of these things. Using DNA-based predictions, a plant’s phenotype can be determined at the seed or seedling stage. Genomics techniques can also be “used to reduce the generation time thereby enabling combinations of desirable traits to be combined on a timescale that is more similar to annual crops.” Below are summaries of specific areas discussed in the paper for using genomics in perennial crop breeding programs:

  • Reduction of Generation Time – This can be done using transgenic technology in ways that do not result in transgenic (GMO) cultivars. One method uses virus-induced gene silencing, in which a host plant is infected with “a virus that is genetically modified to carry a host gene;” the host plant then “attacks itself and uses its own endogenous system to silence the expression of one of its own genes.” Early flowering in apples has been induced after seedlings were inoculated with apple latent spherical virus that expresses a flowering gene derived from Arabidopsis thaliana.
  • Genetic Modification – Advances in genomics have brought us transgenic technology, and several commercial crops have been genetically modified using this technology. Most of them are annuals, but one perennial in particular, SunUp papaya, has been a major success. Its resistance to ringspot virus rescued the papaya industry from a devastating pathogen that “almost completely destroyed the industry in Hawaii.” Consumer disapproval, however, poses a major obstacle to commercial production of genetically modified organisms, and unless this changes, “their widespread use is unlikely.”
  • Marker-Assisted Selection – This is the “primary use of genomics in breeding.” The time between initial plant crosses and the introduction of a new cultivar can be dramatically shortened when genetic markers are used to determine the phenotypes of adult plants at the seedling stage. This technology is also useful when crossing domesticated plants with wild relatives, since genetic markers can be used to determine when desired traits are present in the offspring.
  • Ancestry Selection – After crosses with wild relatives, offspring may “perform poorly because wild germplasm often harbors numerous traits that negatively affect performance.” To overcome this, the offspring is crossed with cultivated plants until undesirable traits are eliminated. This is called backcrossing. Using marker-assisted selection, breeders can “select a small number of offspring in each generation that carry both the desired trait from the wild and the most cultivated ancestry.”
  • Genomic Selection – The success of marker-assisted selection is greatest when used for traits that are controlled by one or a few genes. However, many traits involve a complex set of genes. Genomic selection is a new technique that “uses dense, genome-wide marker data to predict phenotypes and screen offspring.” It is “especially useful for predicting complex traits controlled by many small-effect genes.” Genomic selection is in its infancy, so there are kinks to work out, but it is a promising technology for perennial crop breeding efforts.

The use of genomics will not replace every aspect of traditional perennial crop breeding and “should be viewed as a potential supplement…rather than a substitute.” Geneticists and plant breeders are encouraged to work together to develop and implement these technologies in a concerted effort to improve the crop plants that help feed the world.

"Despite the remarkable phenotypic and genotypic diversity in bananas," the Cavendish banana is responsible for the "vast majority" of banana production. (photo credit: wikimedia commons)

“Despite the remarkable phenotypic and genotypic diversity in bananas,” the Cavendish banana is responsible for the “vast majority” of banana production. (photo credit: wikimedia commons)