Lee asked for some more details on that sexually deviant little fish the Amazon molly (Poecililia formosa). It not only violates leviticus 18:23 , but also laughs at god's hybridisation laws (Lev 19:19) so I’m happy to oblige.
To remind you, they are an all female species that require males of closely related species to reproduce – hence their naming after the tribe of Greek mythology. They are native to the area between the Rio Grande and Tuxpan in Northeast Mexico. It is thought they arose around 100 000 years ago (about 120 000 generations) through the hybridisation of two closely related species; a female Poecilia mexicana and a male Poecilia latipinna. This is the same genus the popular Guppy (P. reticulata) belongs to. This group are unusual in that fertilisation is internal. The male has a modified anal fin called a gonopodium, that acts as a penetrative organ (techno speak for love pump). This can be a significant proportion of the male’s body length, particularly in the genera Priapichthys and Phallichthys (below). No prizes for guessing what the names mean.
Normally, eggs and sperm contain half the number of chromosomes of normal body cells. Upon fertilisation, the normal number is made up – half from the egg and half from the sperm. P. formosa females however, produce eggs that have a full complement of chromosomes, but require sperm from a closely related species (such as the two ancestral species or P. latipunctata and more rarely P. sphenops) to start off the developmental process, but the overwhelming number of offspring receive no genetic input from the male. They are effectively clones of the female. Not much is known about the mechanism, but penetrating the egg can be enough to trigger division. It has been known for a long time that physically piercing the eggs of some species is enough to trigger the first few cellular divisions of embryo formation.
There are some interesting costs and benefits to this system of reproduction. The benefits include the fact that all members of the species can produce offspring; all off which are identical to the parent. There is also an added bonus that beneficial mutations are more likely to survive. It therefore may be possible for unisexual species to take over a habitat. However, one of the drawbacks is that this species absolutely needs males of competing species. Another is that there is no genetic exchange between clones. This would be bad should a new parasite evolve. Another problem is that deleterious mutations can’t be replaced through sex with non affected individuals. In fact, it is estimated that this species would only survive for 70 000 years before accumulated mutations made it go extinct. This principle is called Muller’s Ratchet.
These is some evidence that in very rare occasions, the males can contribute to the genome of the offspring as individuals are occasionally with three or even four sets of chromosomes (four sets could potentially allow the species to one day become sexual again as chromosomes have to have an identical partner to form normal eggs and sperm). Another rare mechanism is that small sub genomic amounts of DNA may come from the sperm. This could form minchromosomes. This allows parts of these chromosomes to replace mutated genes to be repaired by cutting them out and replacing them. There is at least one report that only the somatic (body) cells receive these minichromosomes – and only a small proportion at that. So in this case at least, the minichromosomes can not be transferred to the offspring. There still seems to be a mystery about how this species still exists. Another simpler explanation might be that the species is constantly being re-created through new hybridisations.
There is also a perceived cost to the parasitized males – they waste sperm and resources mating with the wrong species – a mating that will decrease their reproductive fitness. However, it turns out showing interest in another species actually makes hisown females more interested in him. Apparently this works with women too – show interest in the friend of the one you are interested in and that will make the one you are interested in more competitive..
Finally, some species of shark can do without males altogether – at least in the short term.
To remind you, they are an all female species that require males of closely related species to reproduce – hence their naming after the tribe of Greek mythology. They are native to the area between the Rio Grande and Tuxpan in Northeast Mexico. It is thought they arose around 100 000 years ago (about 120 000 generations) through the hybridisation of two closely related species; a female Poecilia mexicana and a male Poecilia latipinna. This is the same genus the popular Guppy (P. reticulata) belongs to. This group are unusual in that fertilisation is internal. The male has a modified anal fin called a gonopodium, that acts as a penetrative organ (techno speak for love pump). This can be a significant proportion of the male’s body length, particularly in the genera Priapichthys and Phallichthys (below). No prizes for guessing what the names mean.
Normally, eggs and sperm contain half the number of chromosomes of normal body cells. Upon fertilisation, the normal number is made up – half from the egg and half from the sperm. P. formosa females however, produce eggs that have a full complement of chromosomes, but require sperm from a closely related species (such as the two ancestral species or P. latipunctata and more rarely P. sphenops) to start off the developmental process, but the overwhelming number of offspring receive no genetic input from the male. They are effectively clones of the female. Not much is known about the mechanism, but penetrating the egg can be enough to trigger division. It has been known for a long time that physically piercing the eggs of some species is enough to trigger the first few cellular divisions of embryo formation.
There are some interesting costs and benefits to this system of reproduction. The benefits include the fact that all members of the species can produce offspring; all off which are identical to the parent. There is also an added bonus that beneficial mutations are more likely to survive. It therefore may be possible for unisexual species to take over a habitat. However, one of the drawbacks is that this species absolutely needs males of competing species. Another is that there is no genetic exchange between clones. This would be bad should a new parasite evolve. Another problem is that deleterious mutations can’t be replaced through sex with non affected individuals. In fact, it is estimated that this species would only survive for 70 000 years before accumulated mutations made it go extinct. This principle is called Muller’s Ratchet.
These is some evidence that in very rare occasions, the males can contribute to the genome of the offspring as individuals are occasionally with three or even four sets of chromosomes (four sets could potentially allow the species to one day become sexual again as chromosomes have to have an identical partner to form normal eggs and sperm). Another rare mechanism is that small sub genomic amounts of DNA may come from the sperm. This could form minchromosomes. This allows parts of these chromosomes to replace mutated genes to be repaired by cutting them out and replacing them. There is at least one report that only the somatic (body) cells receive these minichromosomes – and only a small proportion at that. So in this case at least, the minichromosomes can not be transferred to the offspring. There still seems to be a mystery about how this species still exists. Another simpler explanation might be that the species is constantly being re-created through new hybridisations.
There is also a perceived cost to the parasitized males – they waste sperm and resources mating with the wrong species – a mating that will decrease their reproductive fitness. However, it turns out showing interest in another species actually makes hisown females more interested in him. Apparently this works with women too – show interest in the friend of the one you are interested in and that will make the one you are interested in more competitive..
Finally, some species of shark can do without males altogether – at least in the short term.
3 comments:
Lee asked for some more details on that sexually deviant little fish the Amazon molly (Poecililia formosa). It not only violates leviticus 18:23 , but also laughs at god's hybridisation laws (Lev 19:19) so I’m happy to oblige.
Thanks... from never knowing of such a fish to becoming an ‘expert’ over night.
Who’d a thought it?
It is thought they arose around 100 000 years ago...
... In fact, it is estimated that this species would only survive for 70 000 years before accumulated mutations made it go extinct.
I see a flaw in your evil-ution ideas... 100,000 is bigger than 70,000 therefore evolution is wrong
Ha ha ha ha ha
Sorry... I’m sure there is a perfectly good reason for this – I probably misread something in what you wrote somewhere or there are some huge ‘error bars’ on this 70,000 year estimate or maybe I should follow the link to Muller’s Ratchet before making silly wise cracks.
It is also weird that there are creatures out there that are getting there kicks from screwing different species and it actually helps them in finding a true mate.
I’m just glad they all survived the Flood due to the loop-hole God left in the ‘kill all life on Earth’ plan :)
Lee
or there are some huge ‘error bars’ on this 70,000 year estimate
Its probably just that on very rare occasions some genes from sperm make it to the embryo
Well you would say that Dr evil-utionist...
Actually, it makes sense - unlike the bible.
Lee
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