Deadly competition between parasites

There’s an interesting paper out in Nature Communications: Coinfection alters population dynamics of infectious disease. The paper is the study of Plantago lanceolata a common weed where I live in the UK. It’s found across Europe to India and Africa, and as an invasive plant in North America, China and Australia. It’s an example of why scientific names are so important, because I’d call it Plantain, but it’s nothing like the plantains that people eat as Musa.

P. lanceolata is host to a mildew, Podosphaera plantaginis. It looks like a white powder on the leaves of plants. In fact it’s burrowing roots into the leaf and growing spores to spread elsewhere above it. The parasite is not good for the plant, but it needs a live host to survive. Yet sometimes infections take a turn for the worse. What causes that?

The key is co-infection. Sometimes multiple strains of parasite infect a plant. When that happens things get much more complex. With a simple infection, the parasite only needs to overcome the plant. However, if a plant has multiple parasites, then they compete with each other. Normally a gentle approach would be enough for a parasite, but when there are multiple infections then a more aggressive attacker can pull resources from its competitors. At least that’s how you’d expect it to work.

To find out if it’s true Anna-Liisa Laine and her team based at the University of Helsinki carried out experiments and field surveys in the Åland Islands, southwest of Finland. This is part of a longer-term study on infection, so they had well-known plant populations to examine. In September (2012) they took a leaf from up to 10 plants per population and examined the DNA. They also infected leaves in the lab, and in garden plots.

I think the most striking result they got is in the graph below. The bars measure spore activity, and green and blue bars measure infections from single strains of mildew. The red is what you find when there’s a coinfection.

Coinfection increases spore activity

Singly inoculated plants shown in blue and green and co-inoculated plants in red. (a) Mean number of spores caught on microscope slides from singly inoculated (3=green and 10=blue) and co-inoculated (red) plants. (b) The proportion of live leaf traps that became infected. Error bars are based on s.e.m. Image by Susi et al.

Anna-Liisa Laine, who led the project said: “Here we find that coinfection by different strains of the same pathogen species completely change infection dynamics. These results are really just scraping the surface of how complex infection dynamics can be under coinfection. In our current work we’ve discovered that ribwort plantain populations in Finland contain hundreds of viruses. We’re now measuring how this within host disease community affects infection dynamics for a wide range of pathogen species.”

You can pick up the paper as an Open Access publication from Nature Communications.

Susi H., Barrès B., Vale P.F. & Laine A.L. (2015). Co-infection alters population dynamics of infectious disease., Nature Communications, DOI: http://dx.doi.org/10.1038/ncomms6975

Extrafloral nectar fuels ant life in deserts

Dorymyrmex planidens ants on an EFN secreting cladode bud of the desert cactus Tephrocactus articulatus.

Dorymyrmex planidens ants on an EFN secreting cladode bud of the desert cactus Tephrocactus articulatus.

Many ant–plant associations are mediated by extrafloral nectaries (EFNs): nectar-producing structures not related to pollination and commonly found on leaves and inflorescences. These sweet secretions represent a critical energy resource for many ant species and constitute the basis for protective mutualisms: by providing ants with food, ants protect plants from herbivores. Although EFN-bearing plants occur in a wide range of habitats and climates worldwide, interactions mediated by EFN-bearing plants are poorly documented in deserts. In a recent article published in AoB PLANTS, Aranda-Rickert et al. show that, in a seasonal desert of northwestern Argentina, biotic interactions between EFN-bearing plants and ants are ecologically relevant components of deserts, and that EFN-bearing plants are crucial for the survival of desert ant communities.

Brilliant bird-brained bryophyte diaspore diaspora…

many mosses

Ernst Haeckel, Kunstformen der Natur. Leipzig and Vienna: Verlag des Bibliographischen Instituts, 1904.
Pin for later

There is an ancient and time-honoured association – maybe co-evolution even – between birds and flowering plants, e.g. in respect of pollination and dispersal of the fruits/seeds of the latter by the former. Now, at the other end of the evolutionary spectrum of the Plant Kingdom, is news of another avian–Plantae link-up as Lily Lewis et al. present evidence for long-distance transport of bryophyte ‘bits-and-pieces’ in the plumage of transequatorial migrant birds.

Bryophytes – a general term that embraces mosses, liverworts and hornworts – are so-styled ‘lower plants’ that have occupied the planet for megamillennia and have many important ecological roles. But, like the other members of the Plant Kingdom, bryophytes are essentially immobile and fixed to one location. This poses problems to any enterprising moss, etc., that wants to boldly go, seek out, occupy and colonise new areas, in order to command resources and help to ensure its survival in the dog-eat-dog jungle that is the natural world.

However, evolution has equipped these cryptogams with a phase of the life cycle that is potentially mobile, the spore stage. Transfer of those spores away from the parent plant – and their subsequent germination, establishment and development into individual bryophyte plants – reduces competition for resources between parent and offspring, and extends the area occupied by that species.

Consequently, exploiting agents that can contribute to wide-ranging dispersal of those spores represents a considerable boost to aspirations of territorial gain for an ambitious ‘lower plant’. But reliance on spores to spread the species can be risky; e.g. if the bryophyte taxon concerned is dioicous and it either doesn’t travel along with another spore that gives rise to, or to a place that already contains, the corresponding male/female gametophyte in the new neighbourhood. Which is why Lewis et al.’s work is of considerable interest because – and despite the headline in Scientific American’s news item on the subject – the bird-assisted moss migration is not really about spores, but diaspores.

Although a diaspore (or ‘disseminule’) can be defined as ‘a reproductive plant part, such as a seed, fruit, or spore, that is modified for dispersal’, the definition is usually broadened to include any plant part that could result in the establishment of a new individual. Thus, it includes not only bryophyte spores, but also fragments of established plants, too.

Sampling the plumage of bird species in their Arctic breeding grounds – prior to their South Pole-ward migration – the team found examples of diaspores not only of bryophytes, but also of green algae/cyanobacteria, and fungi. The presence of these putative propagules amongst bird feathers thus seems to establish this phenomenon as another instance of ectozoochory (transport of plant – and algae/fungi/bacteria! – propagation units on the external surface of an animal).

But just because these passengers may be present at the start of the journey doesn’t necessarily mean that they arrive at the carrier’s destination, which in some cases – such as the red phalarope and the semipalmated sandpiper – is the southernmost tip of South America; e.g. could the disseminules be consumed during preening as a sort of in-flight snack by the birds…?

And – as the investigators recognise – even if diaspores arrive, this doesn’t demonstrate that they are viable and could become established in the new home. But it’s another step towards unlocking the mystery of how the disparate bipolar distributions of certain taxa of bryophytes, etc. could be established and maintained. Whether this counts as ‘blue-skies’ research I’m not sure, but it’s a topic that’s certainly got legs and could well take off!

[And if you’re interested in seeing of some of the pre-publication comments on the bryophyte paper, they can be found online. And for more on the world of moss, I recommend Jessica M. Budke’s blog site. – Ed.]

Light-dark O2 dynamics in submerged leaves of C3 and C4 halophytes under increased dissolved CO2: Clues for saltmarsh response to climate change

Salt marshes undergo periodic flooding being subjected to the carbonate chemistry of the water column twice per day. Predicted CO2 rising will change this carbonate chemistry and thus affect differentially C3 and C4 halophytes.

Salt marshes undergo periodic flooding being subjected to the carbonate chemistry of the water column twice per day. Predicted CO2 rising will change this carbonate chemistry and thus affect differentially C3 and C4 halophytes.

Global warming and climate change, as driving forces of sea level rise, tend to increase marsh submersion periods and also modify the carbonate chemistry of the water column due to the increased concentration of CO2 in the atmosphere. In a study published in AoB PLANTS, Duarte et al. found photosynthetic enhancement due to increased dissolved CO2 for both C3 and C4 halophytes. Transposing these findings to the ecosystem, and assuming increased dissolved CO2 concentration scenarios, these results demonstrated a new ecosystem function for the halophyte community, by increasing the water column oxygenation, thus reinforcing their role as principal primary producers of the estuarine system.

Phylogeny and biogeography of wild roses

Phylogeny and biogeography of wild roses The genus Rosa (with 150–200 species) is widely distributed throughout temperate and sub-tropical habitats from the northern hemisphere to tropical Asia, with only one tropical African species. In order to better understand the evolution of roses, this study examines infrageneric relationships with respect to conventional taxonomy, considers the extent of allopolyploidization and infers macroevolutionary processes that have led to the current distribution of the genus.

The ancestral area reconstruction suggests that despite an early presence on the American continent, most extant American species are the results of a later re-colonization from Asia, probably through the Bering Land Bridge. The results suggest more recent exchanges between Asia and western North America than with eastern North America. The current distribution of roses from the Synstylae lineage in Europe is probably the result of a migration from Asia approx. 30 million years ago, after the closure of the Turgai strait. Directions for a new sectional classification of the genus Rosa are proposed, and the analyses provide an evolutionary framework for future studies on this notoriously difficult genus.

Fougère-Danezan, M., Joly, S., Bruneau, A., Gao, X. F., & Zhang, L. B. (2014) Phylogeny and biogeography of wild roses with specific attention to polyploids. Annals of Botany, December 29, 2014, doi: 10.1093/aob/mcu245

Enhancing the acid-soil tolerance of durum wheat

Enhancing the acid-soil tolerance of durum wheat

Enhancing the acid-soil tolerance of durum wheat

Durum wheat (Triticum turgidum) is an important crop but grows poorly on acid soil because of its sensitivity to soluble Al3+. In contrast, hexaploid wheat (T. aestivum) possesses TaALMT1, a major gene for Al3+ tolerance located on chromosome 4D that is responsible for much of the variation in tolerance observed in this species. Han et al. introgress a fragment of the 4D chromosome containing TaALMT1 from hexaploid wheat into an elite durum cultivar and show that the fragment enhances root growth in acid soil. Since the Kna1 locus is also located on the chromosomal fragment, the ability of leaves to exclude Na+, a trait associated with salt tolerance, is also enhanced.

Where did your cells come from?

If you look at your cells under a microscope, you’ll see nearly all of them have a nucleus, mitochondria and other equipment inside them. Eukaryotic cells, cells with a nucleus, are the basis of all complex life fungi, plants and us. The change from prokaryotic cell to complex cell is profoundly important to evolution of life, but how did it happen?

The favoured explanation has been that an archaeon swallowed a bacterium. The two developed a symbiotic relationship and evolved into eukaryotes. This explanation bothers me slightly because it needs the pair to do a lot of work fast, but I suppose if archaea are eating bacteria millions upon millions of times each day, then they’re making a lot of attempts.

David Baum, a University of Wisconsin-Madison professor of botany and evolutionary biologist, has proposed a new model for eukaryote evolution. His model is inside out and, to a non-biologist like me, it looks plausible.

Baum and his cousin Buzz Baum at UCL, argue that archaea developed protrusions called blebs, little arms if you like. These enabled the cells to interact with their environment better. Along the way they encountered bacteria and started to develop ways to exploit the energy of bacteria, while the bacteria were still outside the cell. The cells that did this better survived more often and reproduced until they had engulfed the bacterium.

Inside-out model for the evolution of eukaryotic cell organization. Model showing the stepwise evolution of eukaryotic cell organization from (A) an eocyte ancestor with a single bounding membrane and a glycoprotein rich cell wall (S-layer) interacting with epibiotic α-proteobacteria (proto-mitochondria). (B) We envision the eocyte cell forming protrusions, aided by protein-membrane interactions at the protrusion neck. These protrusions facilitated material exchange with proto-mitochondria. (C) Selection for a greater area of contact between the symbionts would have led to bleb enlargement and the eventual loss of the S-layer from the protrusions. (D) Blebs would have then been further stabilized by the development of a symmetric nuclear pore outer ring complex (Figure 2) and through the establishment of LINC complexes that, following the gradual loss of the S-layer, physically connected the original cell body (the nascent nuclear compartment) to the inner bleb membranes. (E) With the expansion of blebs to enclose the proto-mitochondria, a process that would have facilitated the acquisition of bacterial lipid biosynthesis machinery by the host, the site of cell growth would have progressively shifted to the cytoplasm, facilitated by the development of regulated traffic through the nuclear pore. At the same time, the spaces between blebs would have enabled the gradual maturation of proteins secreted into the environment via the perinuclear space through glycosylation and proteolytic cleavage. (F) Finally, bleb fusion would have connected cytoplasmic compartments and driven the formation of an intact plasma membrane, perhaps through a process akin to phagocytosis whereby one bleb enveloped the whole. This simple topological transition would have isolated the endoplasmic reticulum from the outside world, driven the full development of a system of vesicular trafficking, and established strict vertical transmission of mitochondria, leading to a cell with modern eukaryotic cell organization. Baum and Baum BMC Biology 2014 12:76   doi:10.1186/s12915-014-0076-2

Inside-out model for the evolution of eukaryotic cell organization. Model showing the stepwise evolution of eukaryotic cell organization from
(A) an eocyte ancestor with a single bounding membrane and a glycoprotein rich cell wall (S-layer) interacting with epibiotic α-proteobacteria (proto-mitochondria).
(B) We envision the eocyte cell forming protrusions, aided by protein-membrane interactions at the protrusion neck. These protrusions facilitated material exchange with proto-mitochondria.
(C) Selection for a greater area of contact between the symbionts would have led to bleb enlargement and the eventual loss of the S-layer from the protrusions.
(D) Blebs would have then been further stabilized by the development of a symmetric nuclear pore outer ring complex (Figure 2) and through the establishment of LINC complexes that, following the gradual loss of the S-layer, physically connected the original cell body (the nascent nuclear compartment) to the inner bleb membranes.
(E) With the expansion of blebs to enclose the proto-mitochondria, a process that would have facilitated the acquisition of bacterial lipid biosynthesis machinery by the host, the site of cell growth would have progressively shifted to the cytoplasm, facilitated by the development of regulated traffic through the nuclear pore. At the same time, the spaces between blebs would have enabled the gradual maturation of proteins secreted into the environment via the perinuclear space through glycosylation and proteolytic cleavage.
(F) Finally, bleb fusion would have connected cytoplasmic compartments and driven the formation of an intact plasma membrane, perhaps through a process akin to phagocytosis whereby one bleb enveloped the whole. This simple topological transition would have isolated the endoplasmic reticulum from the outside world, driven the full development of a system of vesicular trafficking, and established strict vertical transmission of mitochondria, leading to a cell with modern eukaryotic cell organization.
Baum and Baum BMC Biology 2014 12:76 doi:10.1186/s12915-014-0076-2

What I like is that there are steps to bringing the bacterium inside the cell, instead of Pow! it’s there and everything has to develop now. That’s probably an unfair over-simplication of the standard model, but the inside-out model makes sense as each step along the way seems to either use material it already has, or confer a small advantage for survival by itself.

While the event happened unseen billions of years ago, Baum and Baum have some ideas of how they can test the idea. Genetic data could help indicate that an inside out model is more likely than the standard model. Their model predicts that some parts of the cell developed in the opposite order to the standard model, though I’ll admit I don’t understand the details of how “COPII-like coatomers are derived from structural components of the nuclear pore, rather than the reverse”. However, I can see a list of clear predictions that Baum and Baum are making that someone can test, even if it’s clearly not me.

Fossil data would be nice, but highly unlikely, but there is another prediction. If prokaryotes can gain an advantage by developing blebs to interact with bacteria, then it should be possible to see some prokaryotes in the wild that look like the first eukaryote before it engulfed its partner.

Best of all, it’s a very positive paper. Baum and Baum aren’t simply saying everyone else is wrong, they’re proposing new topics to research and new things to study, new ways to look at problems. Even if it turns out they’re wrong, they could be wrong in a really interesting and helpful way.

You can pick up the paper through Open Access from BMC Biology.

Baum D.A. & Baum B. (2014). An inside-out origin for the eukaryotic cell, BMC Biology, 12 (1) 76. DOI: http://dx.doi.org/10.1186/s12915-014-0076-2