Parthenogenesis In Amphibians
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Parthenogenesis Parthenogenesis (; from the Greek + ) is a natural form of asexual reproduction in which the embryo develops directly from an egg without need for fertilization. In animals, parthenogenesis means the development of an embryo from an unfertiliz ...
is a form of reproduction where eggs develop without fertilization, resulting in unisexual species. This phenomenon is closely related with reproductive modes such as
hybridogenesis Parthenogenesis (; from the Greek + ) is a natural form of asexual reproduction in which the embryo develops directly from an egg without need for fertilization. In animals, parthenogenesis means the development of an embryo from an unfertiliz ...
, where fertilization occurs, but the paternal DNA is not passed on. Among amphibians, it is seen in numerous
frog A frog is any member of a diverse and largely semiaquatic group of short-bodied, tailless amphibian vertebrates composing the order (biology), order Anura (coming from the Ancient Greek , literally 'without tail'). Frog species with rough ski ...
and
salamander Salamanders are a group of amphibians typically characterized by their lizard-like appearance, with slender bodies, blunt snouts, short limbs projecting at right angles to the body, and the presence of a tail in both larvae and adults. All t ...
species, but has not been recorded in
caecilians Caecilians (; ) are a group of limbless, vermiform (worm-shaped) or serpentine (snake-shaped) amphibians with small or sometimes nonexistent eyes. They mostly live hidden in soil or in streambeds, and this cryptic lifestyle renders caecilians ...
.


Artificial parthenogenesis

Haploid parthenogenesis has been achieved experimentally in
amphibians Amphibians are ectothermic, anamniote, anamniotic, tetrapod, four-limbed vertebrate animals that constitute the class (biology), class Amphibia. In its broadest sense, it is a paraphyletic group encompassing all Tetrapod, tetrapods, but excl ...
, through approaches that could be considered artificial
gynogenesis Gynogenesis, a form of parthenogenesis, is a system of asexual reproduction that requires the presence of sperm without the actual contribution of its DNA for completion. The paternal DNA dissolves or is destroyed before it can fuse with the egg. T ...
.
Oscar Hertwig Oscar Hertwig (21 April 1849 in Friedberg – 25 October 1922 in Berlin) was a German embryologist and zoologist known for his research in developmental biology and evolution. Hertwig is credited as the first person to observe sexual reproduc ...
first achieved artificial parthenogenesis in frogs in 1911, using eggs fertilized by irradiated sperm. The radiation destroyed the DNA within the sperm, but nearly normal embryos were still produced. Gunther Hertwig repeated this experiment in 1924, using crosses between different frogs. A cross between a toad, ''
Amietophrynus regularis ''Sclerophrys regularis'', commonly known as the African common toad, square-marked toad, African toad, Egyptian toad, African bouncing toad (due to the bouncing motion) and Reuss's toad, is a species of toad in the family Bufonidae. It is found ...
'', and a frog, '' Rana fusca'', would not produce a viable embryo, but fertilization of a toad egg by an irradiated frog sperm would produce a haploid larva. Parthenogenesis has also been induced in '' Pelophylax nigromaculatus'' by pricking an egg with a needle. This method produced
tadpole A tadpole or polliwog (also spelled pollywog) is the Larva, larval stage in the biological life cycle of an amphibian. Most tadpoles are fully Aquatic animal, aquatic, though some species of amphibians have tadpoles that are terrestrial animal, ...
s which metamorphosed into adult frogs, some of which were albino. '' Rana japonica'', '' Pelophylax nigromaculatus'' and ''
Lithobates pipiens ''Lithobates pipiens''Integrated Taxonomic Information System nternet2012''Lithobates pipiens'' pdated 2012 Sept; cited 2012 Dec 26Available from: www.itis.gov/ formerly ''Rana pipiens'', commonly known as the northern leopard frog, is a species ...
'' have all produced viable haploid adults as a result of artificial parthenogenesis.


Parthenogenesis in nature


Origins

Salamanders Salamanders are a group of amphibians typically characterized by their lizard-like appearance, with slender bodies, blunt snouts, short limbs projecting at right angles to the body, and the presence of a tail in both larvae and adults. All t ...
are the oldest known parthenogenic vertebrates. Molecular methods date the origins of unisexual salamanders to the
Pliocene The Pliocene ( ; also Pleiocene) is the epoch (geology), epoch in the geologic time scale that extends from 5.33 to 2.58Pelophylax esculentus The edible frog (''Pelophylax'' klepton, kl. ''esculentus'') is a Hybrid (biology), hybrid species of common European frog, also known as the common water frog or green frog (however, this latter term is also used for the North American species ...
'', the edible frog, is the product of crosses between '' Pelophylax lessonae'' and '' Pelophylax ridibundus''. Similarly, '' Ambystoma laterale'', '' Ambystoma jeffersonianum'', '' Ambystoma texanum'' and ''
Ambystoma tigrinum The tiger salamander (''Ambystoma tigrinum'') is a species of mole salamander and one of the largest terrestrial salamanders in North America. Description These salamanders usually grow to a length of with a lifespan of around 12–15 years. ...
'' have been identified as extant parent species to unisexual salamanders within the same genus. However, mitochondrial evidence suggests that the origins of hybrid '' Ambystoma'', on the maternal line, lie in a relative of '' Ambystoma barbouri''. In spite of this, all extant unisexual species of ''Ambystoma'' share no nuclear DNA with ''Ambystoma barbouri''.


Polyploidy in unisexual amphibians

Polyploidy Polyploidy is a condition in which the cells of an organism have more than two paired sets of ( homologous) chromosomes. Most species whose cells have nuclei (eukaryotes) are diploid, meaning they have two complete sets of chromosomes, one fro ...
, a numerical change in the number of chromosomes, is common in parthenogenic amphibians. Triploidy (having three sets of chromosomes), tetraploidy (four sets of chromosomes) and pentaploidy (five sets of chromosomes) are common in salamanders. In unisexual salamanders these different levels of polyploidy are a result of multiple hybridization events, involving two to four species. '' Ambystoma nothagenes'' is a unisexual, triploid hybrid of '' Ambystoma laterale'', '' Ambystoma texanum'' and ''
Ambystoma tigrinum The tiger salamander (''Ambystoma tigrinum'') is a species of mole salamander and one of the largest terrestrial salamanders in North America. Description These salamanders usually grow to a length of with a lifespan of around 12–15 years. ...
'', while hybrids of '' Ambystoma platineum'' and '' Ambystoma texanum'' have been found to be tetraploid. Most '' Ambystoma'' hybrids are described by how many haploid sets of chromosomes they contain from each of their parent species, reflecting their level of ploidy.


Mortality

Embryonic mortality in parthenogenic amphibians is high. Hatching rates for North American salamander species have ranged from 19.5% to 30.5%. It is speculated that intergenomic exchanges, like crossing over during
meiosis Meiosis () is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, the sperm or egg cells. It involves two rounds of division that ultimately result in four cells, each with only one c ...
, may play a role. Intergenomic exchanges are often lethal due to the fact that chromosomes in unisexual species are
homeologous Sequence homology is the homology (biology), biological homology between DNA sequence, DNA, RNA sequence, RNA, or Protein primary structure, protein sequences, defined in terms of shared ancestry in the evolutionary history of life. Two segments ...
(similar, but less so than
homologous chromosomes Homologous chromosomes or homologs are a set of one maternal and one paternal chromosome that pair up with each other inside a cell during meiosis. Homologs have the same genes in the same locus (genetics), loci, where they provide points along e ...
from within a species). Homologous chromosomes are largely identical, in this case as a result of the chromosome replication.


Modes of parthenogenesis and parthenogenetic-like reproduction in amphibians


Gynogenesis

Gynogenesis Gynogenesis, a form of parthenogenesis, is a system of asexual reproduction that requires the presence of sperm without the actual contribution of its DNA for completion. The paternal DNA dissolves or is destroyed before it can fuse with the egg. T ...
is a form of parthenogenesis where an egg begins to divide only after being pricked by a sperm cell, but without the genetic material of the sperm being used. There are two known mechanisms of gynogenesis. The first is an endomitotic event prior to meiosis, where the number of chromosomes in a cell doubles without cell division taking place. After meiosis each egg has the same ploidy (number of chromosomes) as the mother. This particular parthenogentic mechanism has been observed in unisexual '' Ambystoma'' species as well as ''
Glandirana rugosa The Japanese wrinkled frog (''Glandirana rugosa'') is a species of true frog native to Japan and introduced to Hawaii Hawaii ( ; ) is an island U.S. state, state of the United States, in the Pacific Ocean about southwest of the U.S. ...
''. The second potential mechanism is
apomixis In botany, apomixis is asexual development of seed or embryo without fertilization. However, other definitions include replacement of the seed by a plantlet or replacement of the flower by bulbils. Apomictically produced offspring are geneti ...
, which produces a complete set of chromosomes through mitotic replication. This method has not been observed in any amphibious species. Courtship behavior between females of the same species has been observed in '' Ambystoma platineum'', and has been posited to induce either oviposition of ovulation, though the precise utility of the behavior is unknown.


Mole salamander

Unisexual female
mole salamander The mole salamanders (genus ''Ambystoma'') are a group of Salamandroidea, advanced salamanders endemic to North America. The group has become famous due to the study of the axolotl (''A. mexicanum'') in research on neoteny, paedomorphosis, and t ...
s of the genus ''Ambystoma'' are frequent in the region of the North American Great Lakes. These salamanders emerged about 5 million years ago and are the oldest known unisexual vertebrate lineage. The unisexual female ''Ambystoma'' can sometimes undergo
genome A genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding genes, other functional regions of the genome such as ...
exchange with males from sympatric sexual species.


Hybridogenesis

In
hybridogenesis Parthenogenesis (; from the Greek + ) is a natural form of asexual reproduction in which the embryo develops directly from an egg without need for fertilization. In animals, parthenogenesis means the development of an embryo from an unfertiliz ...
, females of a unisexual species mate with a male of a related species and utilize their genetic material in order to produce offspring. However, in spite of this requirement, the genetic material of the male is not passed on to the next generation. Just prior to meiosis, during mitotic division, spindle fibers attach to the maternal chromosomes, leaving the paternal chromosomes in the cytoplasm. The paternal chromosomes are therefore excluded from nascent eggs, without recombination having typically occurred. In some cases, such as ''Pelophylax esculentus'', there is also endomeiosis prior to cell division, which means that the maternal chromosomes are duplicated and each egg contains identical pairs of chromosomes. Hybridogenesis can be described as a parthenogenetic-like mode of reproduction, since there is no continuing heredity in the paternal line . It has been documented in the European water frog complex of the genus ''
Pelophylax ''Pelophylax'' is a genus of true frogs widespread in Eurasia, with a few species ranging into northern Africa. This genus was erected by Leopold Fitzinger in 1843 to accommodate the green frogs of the Old World, which he considered distinct fro ...
'', which includes three hybridogenic forms.


Kleptogenesis

Kleptogenesis is a sexually parasitic form of reproduction in unisexual organisms, that is often associated with species that are also capable of gynogenetic reproduction. In this reproductive mode unisexual females mate with sympatric males of related species, and genetic material in the paternal line recombines with the maternal DNA and thus is passed on. This mode of reproduction can be seen in numerous, though not all, species of unisexual salamander, particularly salamanders in the genus ''Ambystoma'', and is implicated in the exceptional genetic diversity that exists in those animals.


References

{{reflist, 2 Vertebrate parthenogenesis Amphibians