Tropheus are not polygamists

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Unlike the vast majority of haplochromine cichlids, the Tropheus species of Lake Tanganyika in East Africa mate with only a single sexual partner at a time, a new study has shown.

Maternal mouthbrooding haplochromine cichlids from Lake Malawi and Victoria are polygamous and a single brood of fish can be sired by multiple males, however, new evidence has shown that the Tropheus of Lake Tanganyika have a different reproductive strategy and broods are sired by just a single male.

A group of biologists from the University of Graz in Austria worked with Harris Phiri of the Ministry of Agriculture, Fisheries and Food in Zambia, to assess paternity in broods of Tropheus moorii and found that, with just a single possible exception, all of the broods of Tropheus moorii examined in the study were sired by a single male each, which suggests that the multiple paternity seen in the majority of other Rift Lake cichlids does not occur in members of the Tropheini tribe.

The group's findings, reported in the latest issue of the Proceedings of The Royal Society B, explain how a microsatellite-based paternity test was used to work out which male Tropheus fathered each offspring in a number of broods.

The study used five races of Tropheus moorii caught in Lake Tanganyika, as well as a number of other tropheine species including Simochromis pleurospilus, S. diagramma, S. babaulti, Petrochromis fasciolatus and P. orthogonatus, and some less closely related species, Gnathochromis pfefferi and Ctenochromis horei.

All of the Tropheus broods was fathered by just one male, with only a single brood of Petrochromis fasciatus having fry fathered by two separate males.

The authors believe that Tropheus are so unusual compared to the haplochromine cichlids that sexual selection seems less likely to be responsible for their diversification.

Tropheus produce very small clutches of eggs compared to haplochromines with just 5-20 large eggs laid, which are held in the mouth of the female for over a month until they are large enough to be released to fend for themselves.

The group is also sexually monochromatic, so males and females look almost identical, which means that sexual selection might not work in the same way as it does for sexually dichromatic species, such as most of the haplochromines in which the males are brightly coloured and the females a drab shade of beige.

The authors wrote: "The absence of extra-pair fertilisations that would introduce male reproductive variance into a socially monogamous system, the prevalence of territory size and/or quality over male morphology in determining female mate choice, the males' contribution to reproduction over and above the insemination of eggs, and the similar social roles of males and females possibly underlying sexual monochromatism in the taxon distinguish T. moorii from many other 'modern haplochromines', and make it unlikely that sexual selection played a similarly important role in the diversification of Tropheus as is hypothesised for the haplochrmine radiation of Lakes Malawi and Victoria."

Colour morphsThe Tropheus genus currently contains six described species, which include a number of fish regarded as geographic morphs, but which could be distinct species.

In Malawi and Victoria, in which the species flocks are made up predominantly of brightly coloured haplochromines, divergent female preferences and assortative mating are believed to allow colour forms of a single species to coexist alongside each other (sympatrically) as reproductively isolated populations.

A female with a preference for a male with a red chest and vertical blue bars might only mate with males of that type, and ignore males which have a yellow chest and grey bars. It's this sexual selection that is believed to allow new species to form.

However, Tropheus morphs are usually found allopatrically - separately from each other. "Closely related, but allopatrically distributed, variants of T. moorii mate colour-assortatively both in the field following human mediated admixis and under experimental laboratory conditions", the authors wrote.

"But given the low potential for sexual selection in the species, it is unlikely that female preferences could trigger colour diversification in the absence of geographical isolation and give rise to sympatric 'sister morphs'.

"In the light of the above considerations, it is even questionable whether intersexual selection for male traits could have contributed significantly to the allopatric evolution of the manifold Tropheus variants."

For more information on the study see the paper: Egger B, Obermuller B, Phiri H, Sturmbauer C and KM Sefc (2006) - Monogamy in the maternaly mouthbrooding Lake Tanganyika cichlid fish Tropheus moorii. Proceedings of The Royal Society B, 273, 1797-1802.