BIOLOGY & MEDICINE_Evolutionary Biology Sculpin Liaisons The sculpins of Arne Nolte, head of a research group at the Max Planck Institute for Evolutionary Biology in Plön, near Kiel, are no beauties; yet these unprepossessing fish, first discovered in the Lower Rhine in the 1990s, hold a special fascination for him. After all, these particular sculpins are hybrids, the shared offspring of two species. TEXT HARALD RÖSCH t was an historic moment as, in peror’s plan to link navigation along The artificial waterway between the 1807, the workers drove their shov- the Rhine with the then-French port Scheldt and the Rhine opened a gate- els and pickaxes into the muddy of Antwerp. The fact that their work way to the east for the sculpin Cottus ground near Antwerp, Belgium one was also responsible for the birth of a perifretum. Unfazed by the turmoil and last time. At Napoleon’s behest, new species of fish would have gone wars in Europe in the 19th and 20th I they had created a connection between unnoticed were it not for the fact that, centuries, the small bottom-dwelling the River Scheldt and a branch of the almost 200 years later, ichthyologists fish saw its chance. It let the current Rhine, just 35 kilometers away at this discovered a type of fish previously carry it out of its habitat in the upper point, thus bringing to fruition the em- unknown in the Rhine. reaches of the Scheldt tributaries, Photo: Andreas Hartl 56 MaxPlanckResearch 1 | 14 swam through the new canal, and end- known as bullheads, the sculpins have Sculpin, bullhead, miller’s thumb – the fish ed up in the Rhine estuary. migrated upstream at a rate of five to of the genus Cottus are known by many In fact, Cottus perifretum can’t sur- ten kilometers per year, reaching the names. The males mind their offspring with devotion, guarding the eggs on the underside vive for long in large rivers, as it doesn’t Upper Rhine at Karlsruhe and the low- of large rocks from predators, and fanning like mud at all. The water there also gets er reaches of the Main river. their large pectoral fins to ensure a constant too warm in summer, and there’s not supply of oxygen. enough oxygen. In the Rhine, howev- NO PLACE FOR SCULPINS er, the newcomer from the Scheldt met The fact that these crossbreeds can ap- a close relative, the Rhine sculpin Cot- Arne Nolte has been tracking the spread parently conquer a new habitat with tus rhenanus. Its name is misleading in of the new sculpins, which were first more success than their parent species that Cottus rhenanus doesn’t live in the sighted some 20 years ago in the lower goes against the conventional wisdom Rhine itself, but in the tributaries of the reaches of the Rhine. At that time, fish that has long prevailed among evolu- Middle and Lower Rhine. As with its experts were at a loss, insofar as scul- tionary biologists. “Hybrids used to be more westerly counterparts, however, it pins had never been observed in a sim- seen as slip-ups that weren’t really in- can happen that individual fish get car- ilar habitat, let alone one that had been tended in nature. They were thought of ried down in the main current from the so significantly altered by human activ- as low-value rejects that couldn’t repro- headwaters of the tributaries. Some- ity. “It was like finding brown trout in duce even if they arose in the first where in the Lower Rhine region, short- the Rhine, or carp in a mountain place,” explains Nolte. A prime exam- ly before the river empties into the stream,” says Nolte; but that wasn’t the ple of this is the mule, a sterile creature North Sea, the two species managed to only surprise. Using genetic analysis, arising from the hybridization of a produce offspring despite the unfavor- Nolte verified in 2005 that the new- horse and a donkey. able conditions. comers were the result of crossbreeding, The reason for this attitude lies in Helped by massive ecological chang- or hybridization, as biologists call it. Al- the definition of a biological species as es in the lower reaches of the Rhine, the though they look more like the Scheldt set out by biologist Ernst Mayr in the offspring of this liaison have been parental species on the outside, genet- 1940s: a community of individuals that hugely successful since then, conquer- ically they are a mixture of Cottus peri- reproduce among themselves. All indi- Photo: Andreas Hartl ing almost the entire Rhine. Also fretum and Cottus rhenanus. viduals that can come together to pro- 1 | 14 MaxPlanckResearch 57 left: Cottus perifretum (top) and Cottus rhenanus (bottom) look quite similar to the layperson. Their markings and pigmentation don’t aid identification, since they also vary from habitat to habitat within a species. Hybrids of both species resemble Cottus perifretum. right: Ichthyologists distinguish between the two sculpin species by the degree to which spinelike scales cover the body (made visible here by staining). In Cottus perifretum, this prickling covers the sides and caudal peduncle (top), while in Cottus rhenanus, it occurs only behind the pectoral fins (bottom), if at all. duce viable offspring would then be- mark, but the new sculpins in the Rhine This would suggest that hybridization long to the same species. However, it are just one example of the fact that an- constantly provides for new genetic has since become apparent that Mayr’s imal species, too, regularly intermingle variants, especially between closely re- definition is too restrictive and disre- in nature. Not only that, but Nolte is lated species. “Hybrids are more than gards many clearly different species. convinced that hybrids may play a cen- just accidents of nature; on the con- utionary Biology (right, 2) Botanists were quick to realize that tral role in the emergence of new spe- trary, they may have made a major con- different plant species often interbreed cies. It is estimated that approximately tribution to biodiversity on Earth. Were and yet still generate viable offspring. one out of every ten fish and bird spe- it not for that intermingling, we would Zoologists weren’t so quick off the cies hybridizes with another species. have fewer species,” says Nolte. A BIG SMORGASBORD HOW IS A SPECIES BORN? Nolte hopes the sculpins will show him how hybrids generate new species, but For a new species to develop, the gene flow within a species must be prevent- he began by focusing on the diversity of ed or at least restricted. This is the case when individuals are spatially isolat- European fish fauna. This had received ed from the population, by a river or mountain range for example (allopatric only cursory attention until the 21st speciation). The two groups then evolve differently so that, given sufficient century, the first comprehensive and time, two different species emerge. Scientists have often observed this form systematic compilation of European fish of speciation in birds. being published in 2007. Little wonder, However, new species can also develop without spatial separation. In a then, that the nature of the relation- given population, individuals with an extreme characteristic may be ideally ships between sculpin populations had adapted to the environment, such as when small individuals specialize in one attracted little attention. Ichthyologists food type and large individuals in another. In this kind of sympatric speciation, had often lumped the sculpins of Cen- the gene flow is inhibited if the two groups prefer their own type and no lon- tral Europe together in one basket, re- ger reproduce with the other. This process has been well documented partic- ferred to as the Cottus gobio complex. ularly for fish species and insects. Working at the time with Maurice Kot- Genetic exchange between populations can also be prevented if, for telat and Jörg Freyhof at the Alexander example, they specialize in different habitats or develop different courtship Koenig Research Museum in Bonn, behavior, mate at different times, or if the hybrids are inferior to both orig- Nolte identified three different species inal populations. out of that conglomeration, all of which occur in the Lower Rhine region and in directly neighboring rivers. Photos: Jörg Freyhof – Leibniz Institute of Freshwater Ecology and Inland Fisheries, Berlin (left, 2), Jie Cheng MPI for Evol 58 MaxPlanckResearch 1 | 14 BIOLOGY & MEDICINE_Evolutionary Biology » Without hybrids, there would be fewer species on Earth. Arne Nolte has had an interest in fish cases, biologists study populations When hybrids reproduce, parts of the since his childhood years. Cichlids, that have already been diverging for genotypes from both parent species are loach, scalar, killifishes – his aquariums thousands or even millions of years. mixed together, and specific mutations were a veritable “Who’s Who” of pop- “Our hybrid lineage is probably no that affect the viability of the hybrids ular pet fish. His inventory comprised more than 200 years old. That’s quite can occur. Sometimes, for example, up to 40 different tanks at its height; unique,” says Arne Nolte. selfish genetic elements called transpo- even the vegetables had to be moved sons are activated. These are jumping out of his parents’ greenhouse to make HYBRID FITNESS DNA segments that are integrated at room for the fish. It was already clear some point in the genome and inter- to the budding biologist that he want- First, the researchers wanted to clarify rupt genes. In many animal species, ed to work with fish. Since his univer- whether the offspring of the hybrid male hybrids are less viable because sity in Oldenburg offered no such op- sculpins inherit a genetic handicap.