Ch 15 Mutualisms

Mutualisms are interactions between species that benefit individuals of both species.

Mutualisms are interactions between species that benefit individuals of both species. However, they exemplify not altruism, but reciprocal exploitation, in which each species obtains something from the other.  

-Mutualistic interactions are widespread and important. More than 80% of terrestrial plant species form obligatory associations with certain fungi, called arbuscular mycorrhizal fungi, that include many common mushrooms.  

The fungi penetrate the roots

 The fungi penetrate the roots, supply the plant with water and mineral nutrients from the soil, and receive sugars and lipids from the plant. Many symbiotic bacteria in the human and other vertebrate microbiomes are critical in sustaining health. -Another example involves corals, which have a mutualistic relationship with endosymbiotic algae. "Coral bleaching" occurs when seawater is too warm, causing corals to expel their endosymbionts, resulting in coral turning completely white. Some mutualisms have arisen from parasitic or other exploitative relationships.  

Yuccas (Yucca) are pollinated only by female yucca moths

Yuccas (Yucca) are pollinated only by female yucca moths which carefully pollinate a yucca flower and then lay eggs in it The larvae consume some of the many seeds that develop.  

-Some of the closest relatives of Tegeticula simply feed on developing seeds, and one of these species incidentally pollinates the flowers in which it lays its eggs, illustrating what may have been a transitional step from seed predation to mutualism (Fig. 15.17B). 

Figure 15.17 Mutualisms may result in extreme adaptations.

Figure 15.17 Mutualisms may result in extreme adaptations. (A) Yucca moths of the genus Tegeticula not only lay eggs in yucca flowers, but also use specialized mouthparts to actively pollinate the flowers — as the gray moth clinging to the flower's pistil is doing. The moth then inserts eggs into the flower's ovary. (B) A phylogeny of the yucca moth family, showing major evolutionary changes. The genera other than the "habitual pollinators" Parategeticula and Tegeticula are seed predators, some species of which (in Greya) incidentally pollinate the flowers in which they lay eggs. Intimate mutualism evolved in the ancestor of Tegeticula and Parategeticula, and cheating later evolved twice in Tegeticula. 

 

There is the potential for conflict within mutualisms

There is the potential for conflict within mutualisms because a species that cheats by exploiting its partner without paying the cost of providing a benefit in exchange is likely to have a selective advantage.  

-Several possible factors can reduce the fitness of cheaters, and thus maintain a mutualistic relationship. One is simply punishment of cheaters ("sanctions"), to prevent overexploitation. Another possibility is that one or both partner species may be able to choose to reward the most cooperative or beneficial individuals of the other species, or exclude cheaters. 

Yet another possibility is that selection will favor noncheating

Yet another possibility is that selection will favor noncheating, or honest, genotypes if the individual symbiont's genetic self interest depends on the fitness of its host or partner.  

-This will be the case if there is a long-term or permanent association between individuals, restricted opportunities to switch to other partners or to use other resources, or vertical transmission of endosymbionts from parents to offspring. For example, the Buchnera bacteria that live in the cells of aphids and are vertically transmitted are beneficial mutualists.  

Mutualisms are not always stable over evolutionary time

Mutualisms are not always stable over evolutionary time: many species cheat. For instance, many orchids secrete no nectar for their pollinators; some of them, in fact, deceive male insects that accomplish pollination while attempting to copulate with the flower (see fly orchid in Figure 3.21).  

-Two lineages of yucca moths that have evolved from mutualistic ancestors do not pollinate, and they lay so many eggs that the larvae consume most or all of the yucca seeds (“cheaters” in Fig. 15.17B). 

A common reason for the breakdown of a mutualism

A common reason for the breakdown of a mutualism is that one partner adapts to another species that also provides a benefit. 

-A phylogenetic study of seed plants found that their common ancestor was associated with arbuscular mycorrhizal fungi and that this has persisted in most lineages. However, the association has been lost in at least 25 lineages, almost all of which depend on different root-associated fungi or have become carnivorous (e.g., Venus flytrap, Dionaea muscipula) or parasitic on other plants (e.g., mistletoes). 

 

Continuing the theme of extreme adaptations resulting from mutualism

Continuing the theme of extreme adaptations resulting from mutualism, there is the famous example of Darwin's interpretation of an orchid specimen from Madagascar with a nectar tube reaching 30 cm in length, which he received in 1862. The pollinator was unknown.  

-Darwin predicted that the pollinator was a moth with an extremely long tongue. A sphinx moth was identified as the pollinator in 1903. Their probosces are 6.5 cm longer on average that those on in Africa. Why did the orchid's nectar tube and the moth's proboscis become so long? 

The answer is that mutualism often is permeated with conflict.

The answer is that mutualism often is permeated with conflict. Darwin argued that natural selection would cause the insect species to evolve a proboscis long enough to reach the nectar. But why would a very long nectar tube be advantageous to the plant?  

-Darwin suggested that a very long nectar tube would force the insect to press its head deeply into the flower and necessarily pick up and deposit pollen. If the insect's proboscis were longer than the tube, its head would not contact the pollen and the plant would not achieve reproduction 

So, Darwin suggested, there may be an ongoing race

So, Darwin suggested, there may be an ongoing race, in which the plant matches any elongation of the proboscis with an equal or greater elongation of the nectar tube. 

-One hundred forty-seven years after Darwin presented this hypothesis, two research teams tested and confirmed it, using other plant-pollinator associations that are similarly extreme. Anton Pauw and collaborators, studying a South Africa iris with a long corolla tube and a fly with an equally long proboscis, found that flies with longer probosces consume more nectar and that longer-tubed plants receive more pollen. 

Partly because of genomic studies mutualism….

Partly because of genomic studies, mutualism is increasingly recognized as an important basis for adaptation and the evolution of biochemical complexity. 

- The best-known examples are the evolution of mitochondria from proteobacteria and of chloroplasts from cyanobacteria. When a new "compound" organism is formed from an intimate symbiosis, the subsequent evolution of both genomes is affected through endosymbiotic gene transfer (EGT). For example, mitochondrial genomes can encode less than 10% of the mitochondrial proteins (the remainder are found in the nuclear genome).  

Chloroplasts have fewer than 10% as many genes as free-living cyanobacteria

Chloroplasts have fewer than 10% as many genes as free-living cyanobacteria, but many of the original cyanobacterial genes have been transferred to the plant nuclear genome. These genes may account for as many as 18% of a plant's protein-coding genes.  

-Some mutualistic symbioses provide one or both partners with new capabilities. Bacteria and other microbes have formed intimate mutualisms with diverse multicellular organisms, especially animals, which lack the ability to synthesize essential amino acids and vitamins but can obtain some of these nutrients from their microbial partners.

Some extreme associations are in sap-sucking homopteran insect

Some extreme associations are in sap-sucking homopteran insects (aphids, leafhoppers, cicadas, and relatives), which derive different amino acids from as many as 8 different types of coexisting symbionts. 

-Almost all animals and plants carry a huge variety of bacteria and other microbes, referred to as the host's microbiota or microbiome. A human carries 1014 to 1015 microbial cells, far more than the number of human body cells. The great majority are in the large intestine, although there are also distinct biotas in the mouth, nose, and other locations.  

Sequencing RNA genes in samples

Sequencing RNA genes in samples from the gut has shown that humans, ruminant mammals such as cows, and termites carry more than 1,000 microbial species, whereas honeybees have about 10. An individual acquires its microbes from the outside environment and other members of its species.  

-There is considerable variation in the microbial species composition among individual humans, and over time within individuals, but some bacterial species are consistent, and they are related to bacteria in other species of apes—suggesting a long evolutionary history of association. 

Studies of humans, as well as experimental addition of microbes to mice

 Studies of humans, as well as experimental addition of microbes to mice reared in sterile bacteria-free conditions, have shown that this microbiome is critically important for health.  

-Bacteria-free animals suffer from impaired function of the circulatory, hepatic (liver), respiratory, digestive, and endocrine systems. They have lower metabolic rate, higher cholesterol, and a much reduced immune system, especially in the intestinal epithelium. They are more susceptible to infection by Shigella and other dangerous bacteria. Animals lacking gut bacteria require 30% more caloric intake to maintain body mass 

Many resident species of bacteria produce chemicals

Many resident species of bacteria produce chemicals that exclude invading species such as Clostridium difficile; bacterial therapy has been used to control infections by this dangerous bacterium. (Clearly, these are very good reasons to minimize the use of antibiotics and antibacterial products: e.g. Firmicutes is gram positive and susceptible to antibiotics)  

-Mammals depend on a certain bacterial species to regulate the layer of mucus that protects intestinal cells against invasion by other bacteria.  

 

Animals have also harnessed microbes to access new resource

Animals have also harnessed microbes to access new resources: ruminant mammals and termites can digest cellulose, thanks to their microbiome. 

- Humans and most other mammals rely on gut bacteria to digest polysaccharides (Bacteroides, Firmicutes). Some Japanese people have a gut bacterium that, by horizontal gene transfer, acquired from marine bacteria the ability to break down a certain polysaccharide that occurs in seaweed, which is often eaten in Japan. 

The great diversity of species in a host's microbiome

The great diversity of species in a host's microbiome, and its ability to resist invasion by certain other species, result from the population dynamics of the various species and how they affect each other, especially by competition.  

-These dynamics are familiar to ecologists who study similar processes in communities of plants or birds. Just as simple mutualisms between plants and pollinating animals arise from individual selection of selfish genotypes, so do they arise among species in a community of plants or in a community of gut microbes. 

 

 In return the Buchnera bacteria receive the non-essential amino acids from their aphid host.  

Some biologists have urged that we think of the ensemble of microbial

Some biologists have urged that we think of the ensemble of microbial species as an “unrecognized organ” or a unified entity, a holobiont. This may well describe the evolution of hosts with vertically inherited endosymbionts, such as aphids with their Buchnera bacteria, which have evolved to depend on each other completely.   

 In return the Buchnera bacteria receive the non-essential amino acids from their aphid host.  

-But the idea doesn't hold for most other cases, because the variation among different hosts and their microbial associates is not inherited to any significant extent: each individual host acquires most of its microbial species from the environment. 

Figure 15.19 Symbionts associated with a host species

Figure 15.19 Symbionts associated with a host species (here, an insect) may coevolve with the host if they are vertically transmitted from one host generation to the next (A), but not if they are acquired from the environment each generation (B). At left, a symbiont lineage is occasionally lost or gained. Some symbiont lineages change color and form, illustrating evolutionary changes that may enhance a mutualism and form an association that might be called a holobiont. At right, symbionts in the environment colonize each generation of hosts. A host is suitable for only certain microbes that can successfully colonize. The microbes do not form lineages associated with the host genealogy, so there is little opportunity for coevolving and forming a mutualistic association. However, the two lower insects show that microbial assemblages in closely related hosts may be similar because the hosts have similar characteristics. (From N. A. Moran and D. B. Sloan. 2015. PLOS Biol 13: e1002311.)