Interspecific Brood Care

I’ve been thinking about a few posts on cichlid evolution at the moment, but don’t have too much time. These fish make Darwin’s finches look like absolute beginners in evolutionary terms. Lake Malawi alone has at least 500 species and many geographical variations – all thought to have evolved from a single (maybe 2) common ancestor(s) in a few million years. For anyone with some spare time, this site provides some photos of them. A small section on trophic (feeding) adaptations can be found here.

Anyway, as a short post, I would like to introduce some Central American cichlids. Parachromis dovii is a rather large and nasty predator. One problem it faces is the predation of its fry by other fish. Next up is Theraps (aka Hypsophorys, aka Copora) nicaraguense. This is a medium sized fish, but is easily bullied and can be out competed for breeding sites by the aggressive Archocentrus nigrofasciatus. A 12 cm male is a very big fish for this species and I have had 6-8cm fish attack me and draw blood when looking after their fry. There is another smaller species, Neetroplus nematopus, which by all accounts is even more aggressive. So, T. nicaraguense has a problem, these little fish get all the good breeding sites. The solution is rather unexpected. See if you can guess: P. dovii is a major predator of N. nematopus and A. nigrofasciatus………


Give up?

T. Nicaraguense helps bring up the fry of P. dovii and is therefore tolerated by them. Their own fry also benefit from the numbers of P. dovii fry in the area – a predator is more likely to grab one of them as they are more numerous. Som studies estimate that P. dovii would face extinction in some areas if it was not for this interspecific co-operation. There are some costs though, as P. dovii will also eat some juvenile T. nicaraguense too. Thre have also been some recorded incidents of T. nicaraguense “cheating” on the deal too. However, that is evolution for you – cheating can be of benefit to the cheats until their numbers rise and make it disadvantageous – if P. dovii numbers drop, so do T. nicaraguense numbers. Natural selection will then start to select against the cheats and a new equilibrium will be reached – this may then allow another cycle of cheating……

There are obviously some implications for the evolution of moral behaviour here – by helping others, you are actually being selfish and helping yourself. This is something the religious just don’t get – they think being selfish means that you kill and eat every thing – not cooperate.

Sexually Deviant Monosexual Fish

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.