Biological Sciences

One of my ongoing research projects include army ants so I would like to make a brief introduction to that topic.

What are army ants? 

The term army ants is used to describe ant species that present specific behavioural and morphological traits.

These traits are:

Obligate collective foraging and group predation. They form aggressive predatory foraging groups called "raids" that gather all the food that they can find.

Nomadism. These ants don't have permanent nests, instead they form temporary ones called "bivouacs". The army ants stay in the bivouac for a few weeks and they leave when the queen comes out.

Colony fission. Army ants form new colonies when existing ones divide. Young queens with a group of workers can follow a separate way and become another population. Therefore, queens don't need their wings to disperse and are wingless. Males on the other hand need wings that help them find another colony and mate with the queen. The workers of the colony decide whether the male will mate with their queen. Thus, it is apparent that the army ants have a unique mating system with sexual selection and polyandry.

The combination of these traits have been referred to as the army ant syndrome (Brady 2003).

The army ants can be divided in two lineages: the New World army ants and the Old World army ants. The main representatives of the New World is the subfamily Ecitoninae, and of the Old World the subfamilies Aenictinae and Dorylinae.

The worldwide distribution of the main army ants subfamilies.

These two lineages were considered to have evolved independently but after the phylogeny of Brady (2003), there is strong evidence that they all shared a common ancestor in Godwana before South America and Africa break up.

Why study army ants?

Army ants have a strong need of food and as a result they are mostly present in tropical habitats. That means that their role in the ecosystem is extremely important and many species assume a role of top predators. So, their importance on the ecosystem is a really interesting aspect to study.

In addition, they can be a model organism to study sexual selection and polyandry, their evolution and the consequences they have in eusociality.

References

Brady, S.G. (2003). Evolution of the army ant syndrome: The origin and long-term evolutionary stasis of a complex of behavioral and reproductive adaptations. PNAS 100 (11): 6575-6579

Franks, N.R., Hölldobler B. (1987). Sexual competition during colony reproduction in army ants. Biol J Linn Soc 30: 229-243

Inspired from  the new tumblr blog fuckyeahconvergentevolution.tumblr.com I wanted to make a convergent evolution post. This is an example I like to use often in my presentations because it portrays convergent evolution remarkably, but I will get to that later on.

This example includes three species of lizards found in and around White Sands in New Mexico: Eastern Fence Lizard (Sceloporus undulatus), Little Striped Whiptail (Aspidoscelis inornata), and Lesser Earless Lizard (Holbrookia maculata). The area is full of white sand so these three species have independently evolved white (blanched) phenotype for camouflage. The sand there, is derived from gypsum and is formed in less than 6,000 years ago! This means that these new phenotypes evolved rapidly from a brown ancestral phenotype (each one from a different ancestor) that inhabits the surrounding desert.

Mutations associated with blanched coloration in White Sands lizards. (A) Blanched morphs from white sands on top and dark morphs in ancestral dark soil habitat on bottom. (B) Amino acid schematic of the melanocortin-1 receptor (Mc1r); replacements statistically associated with coloration in the focal taxa are shown in red.

The figure above summarises a recent study (Rosenblum et al. 2010). You canssee the two varieties of the three species in A. They all evolved a blanched phenotype while the varieties in the surrounding dark soil habitat are still brown.

Here comes the amazing part. The above study found the genetic basis of this convergence which showed that they are all related to a single amino acid replacement. The amino acid replacement is different in the three species, but it is found in the same gene (Mc1r)! All the aa replacements occur in the transmembrane regions, which are the most important for the structure and function of a TM protein.

Mc1r is a hormone receptor on the surface of melanocytes and is involved to melanogenesis. More specific, it is known to induce the accumulation of cAMP which results in the activation of other genes involved in pigmentation. The wild-type form of Mc1r produces dark pigmentation but when inactivated it results in lighter colours. Other animals have Mc1r mutations too, with the most common example the one of humans where they cause ginger hair. 

In this study they tested the genetic basis of this convergence.

First, they found a strong correlation of the three amino acid mutations with the blanched colouration in the white sand habitat.

The next step was functional assays, which showed that in two of the three species (S. undulatus and A. inornata) the Mc1r mutants reduced the proteins activity. This wasn't the case for H. maculata, even though it exhibited the strongest correlation. This could be just a random association or it could mean that there is an unknown mechanism of Mc1r.

Finally, they proceeded to genetic analysis according to which the white allele is dominant in S. undulatus while it's recessive in A. inornata. The dominance is most likely selected by evolution as it produces a protein that prevents the wild type form from integrating into membranes. This difference has an impact on the gene pool of these two species. It is natural that in A. inornata the dark coloured wild-type form is removed by selection immediately as it produces dark pigmentation even in heterozygous individuals, whereas in S. undulatus the wild-type gene remains in the gene pool in higher frequency.

This is an amazing textbook example of convergence not only at the phenotypical level but at the molecular level as well and it's not even my favourite example of convergent evolution (i will probably make other posts too).

Here is a picture (unknown source) to see how big the selective pressure for the white colouration is. 

Reference

The Mediterranean monk seal (Monachus monachus) is one of the 33 species of the pinnipeds in the world. It was first described by Johann Hermann in 1779 from a single specimen of Strasbourg's museum who gave the name Phoca monachus  from the greek word "μοναχός" which translates to monk. Fleming in 1822 first used the name Monachus as the genus, which now includes two other pinniped species: the Hawaiian monk seal (Monachus schauinslandii) and the Caribbean monk seal (Monachus tropicalis). 

The Caribbean monk seal is considered extinct as it was latest seen in 1952 (Adam 2004).

The Hawaiian monk seal is critically endangered and it's population is less than 1400 (Baker & Johanos 2004)

The Mediterranean monk seal is considered as the most endangered mammal of Europe (critically endangered in IUCN) and one of the 6 most endangered animals in the whole world, with less than 600 individuals (Johnson et al. 2006).

Monachus monachus is now found on inaccessible coastlines and avoids human contact. Historically it used to occupy the whole Mediterranean sea, North-West African coastline, Black sea, Canary islands and Azores but now due to human hunting from the roman empire in the middle ages it is limited to specific areas (see map above).

The most noteworthy population of Monachus monachus lives in the eastern Mediterranean region in Greek and Turkish coastline which consists of 300-350 individuals, more than half of the species population. Although it is really difficult to estimate it's geographical distribution in that area (Brasseur et al. 1997) classical methods (satellite imaging, catch-release) haven't been used yet due to their choice of inaccessible terrestrial habitats.

The main threats of the Mediterranean monk seal as estimated by Androulaki et al. 2006 and Johnson et al. 2006 are:

Deliberate killing (by fishermen)

Habitat loss and deterioration (coastline habitats) from human activity (tourism, industrialization)

Lack of food due to overfishing 

Accidental deaths through entanglement in fishing gear

Pollution

Random events such as epidemics, oil spills, toxic algae

Inbreeding depression (Pastor et al. 2004)

Conservation actions include: creation of special conservation areas, scientific researches to identify and monitor it's habitat, decrease of human caused deaths and effective coexistence with humans (fishermen) and public awareness.

References

Adam, P.J. 2004. Monachus tropicalis. Mammalian Species 747:1-9

Androulaki, E., A. Chatzispyrou, S. Adamantopoulou, P. Dendrinos, A. Komnenou, I. Kuiken, E. Tunta, & S. Kotomatas. 2006. Investigating the causes of death in monk seals, stranded in coastal Greece (1986-2005) In: I. Kuklik, ed. 20th Conference of the European Cetacean society

Baker, J. D. & T. C. Johanos. 2004. Abundance of the Hawaiian monk seal in the main Hawaiian Islands. Biological Conservation. 116: 103-110.

Brasseur, S., Reijnders, P., & Verriopoulos, G. 1997. Mediterranean monk seal Monachus monachus. In P. Reijnders, G. Verriopoulos, & S. Brasseur (Eds.), Status of Pinnipeds Relevant to the European Union (IBN Scientific Contributions), 8, 12-26.

Johnson, W. M., Karamanlidis, A. A., Dendrinos, P., de Larrinoa, P. F., Gazo, M., González, L. M., et al. 2006. Monk seal fact files: Biology, behaviour, status and conservation of the Mediterranean monk seal, Monachus monachus. The Monachus Guardian. Retrieved 20 October 2006 from www.monachus-guardian.org

T. Pastor, J. C. Garza, P. Allen, W. Amos & A. Aguilar. 2004. Low Genetic Variability in the Highly Endangered Mediterranean Monk Seal. Journal of Heredity. doi: 10.1093/jhered/esh055

Useful sites

http://www.monachus-guardian.org/index.php

Bathynomus giganteus  is a deep sea isopod species first described by the french zoologist Alphonse Milne-Edwards(1879). It is the largest known isopod crustacean with an average length of 46 cm and the largest detected being around 76 cm long. It is a deep sea predator and scavenger and it feeds on cephalopods, sponges, crabs, nematodes and polychaetes. It is found in depths of 300 to 2000 meters in the Caribbean(mostly), the southern Atlantic and the Indian ocean in muddy surfaces and near cold seeps. 

Here is a picture

They are adapted to long periods of starvation with most commonly known the giant isopod that survived 5 years without food in captivity.

All the other species of the Bathynomus genus live in deep sea and exhibit gigantism(from 10 to 30 cm). This form of gigantism is similar to the one in islands where giant sizes of small animals are apparent(the island rule, Van Halen 1973) and is called deep sea or abyssal gigantism. It is believed that the deep sea and shallow sea are analogous to the islands and mainland, mostly because of the various observations of deep sea giants such as the giant squid, the giant amphipod and the Japanese spider crab.

The question that comes to mind now is why they developed this trait. One possible explanation is the scarcer food resources in the deep sea resulting in a lower rate of development(delayed sexual maturity) therefore growing for more time and reaching larger sizes. Another explanation is the low temperatures(Timofeev, S.F. 2001) which lead to bigger sizes for a lower surface area to volume ratio keeping them warmer(analogous to Bergmann's rule).

The opposite(deep sea dwarfism) is also observed e.g. in gastropods(McClain, C.R. et al. 2005).

In addition to gigantism Bathynomus giganteus has another distinct feature, it's vision. All the species of Bathynomus have relatively large eyes contrary to the general rule of eye reduction(even complete loss) in high depths because of the lack of light. Bathynomus giganteus has as much as 4000 facets per compound eye compared to the 40 of common isopods(Chamberlain, C. Steven et al. 1986).

References: 

Chamberlain S.C, Meyer-Rochow V.B, Dossert W.P. 1986. Morphology of the compound eye of the giant deep-sea isopod Bathynomus giganteus. Journal of Morphology

Cocke B.T. 1987. Morphological variation in the giant isopod Bathynomus giganteus (suborder Flabellifera: family Cirolanidae) with notes on the genus. Master's thesis, Texas A&M University

Holthuis L.B. and Mikulka W.R. 1972. "Notes on the deep-sea isopods of the genus Bathynomus A. Milne-Edwards, 1879". Bulletin of Marine Science

McClain, C.R., A. Boyer, G. Rosenberg. 2006 The island rule and the evolution of body size in the deep sea. Journal of Biogeography

McClain, C.R. and Etter, R.J. 2005 Mid-Domain Models As Predictors of Species Diversity Patterns: A Case Study Using Bathymetric Diversity Gradients in the Deep Sea. Oikos