Arxiu d'etiquetes: squamates

ARTHROPODS, CONTENTS, ENGLISH, General, REPTILE, Reptiles: Anatomy and physiology, ZOOLOGY

Immaculate Conception… in reptiles and insects

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December’s bank holidays and Christmas’s holidays have in common in that the Immaculate Conception is celebrated in both. The biological phenomenon in which a female animal reproduces without mating with a male is called parthenogenesis and, even if there isn’t any proof that this could happen to human beings, virginal birth is a widely distributed thing throughout the animal kingdom. In this entry we’ll see how this incredible phenomenon happens and some species in which it appears.

WHAT IS PARTHENOGENESIS?

Parthenogenesis is a type of asexual reproduction in which the offspring comes from a non-fertilized ovum. Without fertilization (union of the oocyte’s and the sperm’s genetic material) the offspring won’t have any part of the father’s DNA (if there is a father). The resulting babies will be genetic copies (clones) of their mother.

532px-Haploid,_diploid_,triploid_and_tetraploidDuring fertilization, when the ovum and the sperm fuse together (both haploid cells, with just one copy of chromosomes, n chromosomes) a new individual is formed with a unique genetic combination, with DNA from its father and its mother (diploid, with two copies of each chromosome, 2n chromosomes in each cell). Triploid (3n) or tetraploid (4n) individuals only appear in asexual hybrid species, and most cases are non-viable. Images by Ehamberg.

In parthenogenetic animals, the lack of paternal genetic material must be compensated because in many species haploid foetuses are non-viable. In these species diploidy (2n chromosomes) is usually re-established through a process called automixis. Yet in some species, haploid individuals with parthenogenetic origins are viable and have no problems in surviving.

It is impossible to pose a general example for asexual reproduction, as it is widely distributed through very different animal groups and there are many cases with many differences among them. Bellow, we’ll present you some examples of different strategies used by animals to reproduce asexually.

HAPLODIPLOIDY IN BEES AND WASPS

Haplodiploidy is a phenomenon that appears in two insect orders, hymenopterans (bees, ants and wasps) and thysanopterans (thrips or stormbugs). In this sexual determination system, if the ovum is fertilized it will develop into a female while, if it isn’t fertilized a haploid male will be born.

Apis_Mellifera_Carnica_Queen_Bee_in_the_hiveColony of Carniolan honey bees (Apis mellifera carnica), a subspecies of hony bee from Eastern Europe. Photo by Levi Asay.

In the honey bee, when the queen bee mates with a drone (male bee), all the diploid individuals (2n) will became females, with DNA combined from the queen and the drone. By contrast, drones are born by parthenogenesis, in which an egg from the queen will develop into a haploid drone (n). This means that the individuals in a bee colony, descendants from the same queen, are much more closely related to each other than regular siblings (drones have 100% of their mother’s DNA). It is believed that this helped to the development of eusocial behaviours in different hymenopteran groups.

CYCLIC PARTHENOGENESIS

This kind of parthenogenesis is found in different invertebrate groups that can alternate between asexual and sexual reproduction during its life cycle depending on the environmental conditions.

1471-2164-14-412-1-lDiagram about the life cycle of a rotifer, in which parthenogenetic asexual reproduction during good environmental conditions is alternated with sexual reproductions with a haploid male during adverse conditions. Image extracted from Hanson et al. 2013.

Some invertebrate groups like aphids, present asexual parthenogenic reproduction from spring until early autumn, when conditions are favourable. During this stage in many populations we find only females that give birth to more females.

Fast motion video in which we can see how the aphids take advantage during good weather conditions to increase fast and efficiently the number of individuals asexually. Video by Neil Bromhall.

When autumn approaches, parthenogenetic females start giving birth to sexual males and females. Both sexes are born by parthenogenesis and have 100% of their mother’s DNA. Sexual winged individuals then disperse to avoid mating with their own siblings. These will mate and females will lay resistant eggs that will survive winter. In spring these eggs will hatch and give rise to a new generation of parthenogenetic females that will start the cycle again.

TRUE PARTHENOGENESIS IN SQUAMATES

The only vertebrates that show true parthenogenesis are the squamates, with about 50 lizard species and one snake being obligate parthenotes. These are unisexual species, all individuals being females that reproduce asexually without the intervention of any male. Also, there are many other species that, even if they usually reproduce sexually, are also able to reproduce asexually when there are no males available (facultative parthenogenesis).

DesertGrasslandWhiptailLizard_AspidoscelisUniparensDesert grassland whiptail lizard (Cnemidophorus uniparens) which, as its scientific name implies, is a parthenogenic species in which all specimens are female. Photo by Ltshears.

There are isolated cases of captive female sharks, snakes and Komodo dragons that have reproduced without fertilization or mating with a male. Yet, this is known as accidental parthenogenesis, because the high mortality of the offspring (surviving between 1/100.000 and 1/million) shows that it is probably due to a failure of the organism, more than an adaptive phenomenon.

ParthkomodoBaby Komodo dragon (Varanus komodoensis) born by accidental parthenogenesis at Chester Zoo. Photo by Neil.

Females from the true parthenogenetic species produce haploid eggs (with n chromosomes) which eventually become diploid (2n chromosomes) by two consecutive division cycles during meiosis (automixis). In species with facultative parthenogenesis, diploidy is achieved by the fusion of the ovum with a haploid polar body that forms during meiosis.

Oogenesis-polar-body-diagramScheme of the formation of polar bodies during oogenesis, which may help parthenogenetic reptiles to regain their diploidy. Scheme by Studentreader.

True parthenogenesis is especially well-known in the Brahminy blind snake (Ramphotyphlops brahminus) and many species of lizards. In these species females generate clones of themselves. Parthenogenetic lizard species (like in amphibians) probably originated from a hybridization event between two sexual species. Many whiptail lizards (genera Cnemidophorus/Aspidoscelis) present unisexual species in which no males exist, from a hybridation process.

Ramphotyphlops_braminus_in_Timor-LesteBrahminy blind snake (Ramphotyphlops braminus), the only known unisexual ophidian, in which all specimens found to date are females. Photo taken from Kaiser et al. 2011.

The species Cnemidophorus uniparens is a parthenogenic unisexual species, which appeared asa result of the hybridization between C. inornatus and C. burti. The resulting hybrid reproduced again with C. inornatus, forming the triploid (3n) parthenote C. uniparens. The presence of triploid, tetraploid, etc. genomes is a common phenomenon between unisexual reptiles, as its hybrid origins sometimes prevents the mixing of genomes. Also, a greater chromosomal variability compensates the lack of genetic recombination.

Despite being unisexual, sexual behaviours have been observed in this species similar to bisexual species. In C. uniparens there are documented sexual behaviours in which one female takes the role of a male and “mounts” another female contacting their cloacae. It is known that mounted females increase their egg production after this fake copula. It is believed that from one year to the other females shift their roles of mounting or being mounted, varying from year to year the number of eggs laid.

Cnemidophorus-ThreeSpeciesThree species of whiptail lizards. The middle one, Cnemidophorus neomexicanus is an unisexual parthenogenic species, originated from the hybridization of two bisexual species, C. inornatus (left) and C. tigris (right). Photo by Alistair J. Cullum.

Even if they are true parthenogenetic species, many of these squamates keep their ability to add new DNA to their offspring. This is due to the fact that if there’s no genetic recombination by the fusion of the ovum and the spermatozoon, there’s a high risk of accumulating genetic mutations detrimental for the species. Yet parthenogenesis allows these species to quickly colonize new habitats, because it is not necessary for two individuals to find each other to procreate, and 100% of the population is able to reproduce.

As you can see, there is a great number of animals that don’t need males nor sex to reproduce. The existence of a similar process in human beings is pretty much improbable (no to say impossible). Besides, if 2000 years ago a woman would have given birth to a baby without fertilization, probably this would have been a girl, because it wouldn’t have been able to acquire the Y chromosome from anywhere. Yet, this doesn’t mean we cannot enjoy the upcoming holidays. Merry Christmas and Happy New Year to everyone!

REFERENCES

The following sources have been used during the elaboration of this entry:

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CONTENTS, ENGLISH, REPTILE, Reptiles: Nowadays diversity, ZOOLOGY

Having no legs doesn’t make you a snake

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With the arrival of good weather it becomes more probable that we go out to the forest to enjoy nature, and the possibilities of finding snakes and other reptiles sunbathing on a stone or running among the grass increase. Snakes are the best known legless squamates, even though there are many other species of lizards which have also lost their extremities during their evolution. In this entry I’ll explain some distinctive characteristics of the three species of legless lizards that we can find in the Iberian Peninsula, the slow worm and the Iberian worm lizards.

LIMBLESS LIZARDS

The loss of legs is an evolutionary phenomenon that has happened more than once in the Squamata order. In fact, currently there are at least nine different lineages of legless lizards (not counting snakes).  In most groups this happens as an adaptation to a subterranean lifestyle (these usually present a short, round tail) or to a life among grass and vegetation (which usually show a long, slender tail).

1Scheltopusik or European legless lizard (Ophisaurus apodus) a limbless lizard of the Anguidae family, photo by Tim Vickers.

Even though technically snakes are also legless lizards, unlike other groups, some ophidian species may pose a potential threat to human beings. This is why it’s important to know how to tell a snake from a legless lizard. There are some characteristics which can help us to differentiate a snake from a non-venomous lizard:

  • Snakes haven’t got movable eyelids, while the rest of lizards do have.
  • Ophidians have no external ear, while in most lizards the auditory channel can be appreciated.
  • Snakes present specialized ventral scales for locomotion, while most limbless lizards have to move with the aid of the irregularities of the substrate.
  • Many legless lizards can shed their tail as a defense mechanism (caudal autotomy) while snakes can’t.
www.public-domain-image.com (public domain image)Picture of a western green mamba (Dendroaspis viridis), a typical ophidian, by Jon Sullivan.

In a previous entry we already explained the different snake species that can be found on the Iberian Peninsula. Below, I’ll present you the three different species of legless lizards that we can find when we go out to visit natural landscapes of our country.

SLOW WORM (Anguis fragilis)

The slow worm is a legless squamate within the anguid family (Anguidae), in which we find the Anguinae subfamily, in which many species have lost their limbs or have them extremely reduced in size. The slow worm’s scientific name, Anguis fragilis, means literally “fragile snake”, referring to its ability to shed its tail to escape predators.

SONY DSCPhoto of a slow worm close to Nismes, by © Hans Hillewaert.

Description

The slow worm is a small lizard with no visible legs, which can grow to 40 centimetres in length. It presents shiny, smooth scales and a small head with a poorly differentiated neck. Unlike snakes, it has movable eyelids, a forked tong and a small tympanic aperture.

Juvenile individuals usually have a golden or silver brown colouration with their sides and belly of a black coloration. Females and juveniles are similarly colored, being ochre with a dark brown or black belly and a black dorsal band, even though their coloration varies a lot.

Slow Worm (Anguis fragilis), seen near Hitchin, Hertfordshire, during the final test of the August GOC walk, on 3 August 2013. It's the first ever reptile I've photographed, and indeed, the first I've seen in the wild! So I was very happy.Female slow worm, photografied at Hertfordshire by Peter O'Connor.

Males are more uniformly colored, with its back and sides of brown or grey coloration, while some older individuals show dark brown spots on their sides which may become of a bluish coloration with age.

6Male slow worm, with distinctive blue spots, by Maria Haanpää.

Habitat and distribution

It’s a widely distributed reptile throughout most Europe, all being found from the Iberian Peninsula, England and Scotland up to Iran and west Siberia, passing through Greece and Turkey.

7Map showing the slow worm's distribution, by Osado.

In the Iberian Peninsula it is found mainly in the northern half, occupying most Galicia, Asturias, Basque Country and Castile and León and the north of Aragon and Catalonia. The slow worm is a common species that goes unnoticed thanks to its inconspicuous customs. We can find slow worms in a wide variety of open habitats, such as grasslands, scrublands and open forests.

8Distribution of the slow worm in Spain, by Lameiro.

Unlike most reptiles, which look for sunny places to warm up, the slow worm has a strong preference for wet and shadowy places, with plenty of low growing vegetation. It usually shelters under stones, tree logs, plastic wastes or small mammal’s burrows.

Male slow worm (Anguis fragilis)A male slow worm on its habitat, on the Netherlands, by Viridiflavus.

Biology and ecology

In the Iberian Peninsula the slow worm is active from the end of February to November, when hibernation starts, during which groups of up to 100 individuals can be found. Mating lasts from middle March up to July, during which males can be found fighting. Their gestation period lasts about three months, they are ovoviviparous species (females produce eggs but babies hatch inside their mothers) and females give birth from 2 to 22 young.

Many different species of reptiles, birds and mammals prey upon this species. As other lizards, the slow worm can shed its tail as a defence mechanism, which continues moving while the rest of the animal flees. The tail starts to regenerate after a few weeks.

10Picture of a slow worm after shedding its tail, by SuperMarker.

Slow worms feed on snails, earthworms, insect larvae and many other small invertebrates, because, unlike snakes, they can’t unhinge their jaws to swallow big prey. This animal has been unfairly persecuted even though it is a helpful species for fields and gardens, as it feeds on many species considered pests for many cultivated plants.

11Photo of a slow worm feeding on a slug, by Biosphoto/Thiebaud Gontard.

WORM LIZARDS (Blanus cinereus and Blanus mariae)

Amphisbaenians (clade Amphisbaenia) are a group of highly specialized subterranean squamates known as worm lizards. Even though externally they resemble some primitive snakes, they are different in that, while snakes first lost their front limbs and their left lung, worm lizards first lost their hind limbs and their right lung. Currently about 180 species of amphisbaenians are known, two of which are found on the Iberian Peninsula: the Iberian worm lizard (Blanus cinereus) and the Maria’s worm lizard (Blanus mariae), both differentiated by distribution and genomic studies.

12Iberian worm lizard in Andalusia, photo by Antonio.

Description

Worm lizards are reptiles strictly adapted to a subterranean lifestyle, with bodies externally resembling that of earthworms. At first it’s difficult to tell the head and the tail apart, which is useful for worm lizards when it comes to escape predators (just as the slow worm, worm lizards can shed their tail, which doesn’t regenerate completely).

SONY DSCIberian worm lizard next to Murcia. Note the similarity between head and tail. Photo by Jorozko.

Adults may measure more than 15 centimetres in length, with some individuals reaching 30 centimetres. The head is blunt and short, with a wide frontal scale to aid them while digging. Their eyes are vestigial (they can only detect changes of light intensity) and are covered by scales, while they have very acute hearing and smell.

14Photo of the head of an Iberian worm lizard, where you can see the scale-covered eyes, by J. Gállego.

Scales are rectangular and are distributed evenly forming rings around their body. Coloration goes from pale pink, to dark purple and brown, and there is no sexual dimorphism between males and females. Like all amphisbaenians, worm lizards can move both forwards and backwards.

15Adult worm lizard next to Cáceres, in which we can see the rectangular and evenly distributed scales. Photo by Mario Modesto.

Habitat and distribution

The two peninsular species of worm lizard are found exclusively in the Iberian Peninsula, except in the north and northeast, from sea level up to 1800 metres of altitude (in Sierra Nevada). The Iberian worm lizard (Blanus cinereus) is more widely distributed, while the Maria’s worm lizard (Blanus mariae) occupies the southwest of the peninsula.

16Distribution map including both Blanus cinereus and Blanus mariae, by Carlosblh.

Worm lizards are found in a wide variety of habitats, from forests of holm oaks, pine trees and oaks to crops, gardens and sandy areas. They have subterranean habits, and usually take shelter under rocks and logs. Like the slow worm, worm lizards prefer humid zones and with soft soil, easy to dig into.

Biology and ecology

Worm lizards are active all year round, even though their activity specially intensifies during spring, summer and after rainy weather. During the day they usually shelter in underground galleries or under logs and rocks. In winter they maintain their body heat, moving through galleries at different depths or staying under sun warmed stones.

P1050134Photo of an Iberian worm lizard next to Cádiz, photo by Jorge López.

Their diet is composed of insects, arachnids and other arthropods found between leaves or underground. Worm lizards are eaten by a great number of terrestrial vertebrates, and their defense mechanisms include: tail scission, escaping to some of their galleries or curling up to form a ball.

Video of an Iberian worm lizard from Albacete, by Encarna Buendia.

The reproduction season goes from February to June, while mating usually occurs between April and May. Females lay a single relatively large egg, which is abandoned buried underground. Incubation period lasts for 69 to 82 days, and the newborn measure between 78 to 86 millimetres.

16Photo of a pair of Iberian worm lizards in a garden near Seville, by Richard Avery.

OTHER LEGLESS LIZARDS

As I’ve already said, apart from the species described above, there are many other groups of limbless lizards over the world. Some of these other groups are:

Scincidae family: A family of chubby, short legged lizards, many of which have no functional limbs. In the Iberian Peninsula we can find two species: the Bedriaga’s skink (Chalcides bedriagai) and the western three-toed skink (Chalcides striatus).

Benny_Trapp_Chalcides_striatus_Spanien
Western three-toed skink, photo by Benny Trapp.

Pygopodidae family: A group of lizards with absent or reduced limbs, related to geckos.

17Photo of a Burton's legless lizard (Lialis burtoni) from southern Australia, by Matt.

Dibamidae family: Legless tropical lizards of subterranean habits.

18Photo of a dibamid called Anelytropsis papillosus, taken from Tod W. Reeder et al.

Anniellidae family: American legless lizards.

19A legless lizard from the Anniella genus, form California, by Marlin Harms.

Even if most legless lizards are harmless, it doesn’t mean we can touch them and handle them in any form we want when we find them in nature. Legless lizards, as most wild animals, are easly stressed by human handling and shouldn’t be handled except for scientific purposes. The best way to enjoy nature is by observing it without disturbing it.

REFERENCES

The following sources have been consulted during the elaboration of this entry:

Difusió-anglès