Partner choice and the formation of species

The choice of a partner and sexual preference play a very significant role in the origin or the extinction of species. Since January, the Leiden Biologist Machteld Verzijden has been conducting research on the sexual preference of the sword tail fish at the Texas A&M University. She has recently received a Rubicon subsidy to carry out this study.

Machteld Verzijden: 'In the first part of my research I will chart two types of sword tails and their hybrids.'

Reproductive isolation
The formation of species is concerned with a mechanism known as reproductive isolation. This mechanism takes care that the different species refrain from sexual contacts among each other. 'This is the reason that a cat will never mate with a dog and that the sexual preference of humans is as a rule not directed towards animals,' Verzijden explains. 'But groups may separate themselves and ultimately create a new species. That can only occur if there are two recognisable groups within the species and the animals of one group no longer go on sexual escapades to the other group.'

Cichlids
The mechanism determining the sexual preference of a species is susceptible to change. Verzijden is primarily interested in this aspect. For her PhD she researched the partner choice of cichlids, a small fish found in Lake Victoria in East Africa. In a relatively short period of time a number of species have developed there but it was unclear what caused this proliferation of species. 'A striking characteristic is that these small creatures select very strictly and mate exclusively with fellow group members which greatly resemble themselves,' says the biologist. 'For quite a while this preference was considered to be innate.'

Adoptive mother
'The special characteristic of these small fish is that the females hatch their eggs inside their mouths. This causes a lengthy contact between mother and the young. In my experiments I exchanged the eggs of different groups, which I then had an adoptive mother of a different group hatch. And what happened to my surprise? At a later age, the fish which had been hatched by the adoptive mother chose their partners from the same group as the adoptive mother. This means that sexual preference is not innate but has been acquired. It may be an explanation for the relatively rapid proliferation of species in Lake Victoria.'

Xiphophorus hellerii

Biodiversity
Verzijden is now involved with the same themes - partner choice and formation of species - in the United States. This time the subject of her research is the green sword tail (the Xiphophorus family), a fresh water fish which occurs in Central-American rivers and is also a very popular aquarium fish. Verzijden: 'Compared to the cichlids, the exact opposite is occurring in these fish. Increasingly fewer species exist. Under the influence of human beings - disposing of their waste in the rivers and lacking good sewers - the sword tail's environment is changing. Consequently their partner choice is becoming less strict, causing the development of fewer 'pure' species and creating crossbreeds or hybrids. In this way diversity is being reduced.

Xiphophorus maculatus

Hybrids
The sword tail's name, at least the male's, derives from its clearly visible sword at the base of the tail fin. This secondary male characteristic is very attractive to the female. Verzijden: 'In the first part of my research I will chart two types of sword tail fish and their hybrids. I want to know where they occur and what they look like. A number of external characteristics are different in the two species, such as their sword lengths, their sizes and the colours of their bodies. In hybrids, all the cards have been reshuffled, as it were, and new variations of characteristics may occur.

Hybrid of Xiphophorus helleri and Xiphophorus maculatus

Video simulation
Verzijden wants to know what exactly attracts the females and on what their choice of partner is based. 'I hope to discover this by means of video simulations,' she explains. 'I will project different species with their different external characteristics on a screen, in order to determine to which fish the females feel most attracted. I will observe which male they stay with longest. I will only look at physical characteristics for the time being. We also know that scent plays an important role, but too little is known on that score as yet. Based on my results, I will design a model enabling me to predict how the population will look in a hundred years' time or even longer.'

(13 May2008/Marl Pluijmen)