Ulrike Strecker

Parallel speciation in Astyanax cave fish (Teleostei) in Northern Mexico.

We investigated differentiation processes in the Neotropical fish Astyanax that represents a model system for examining adaptation to caves, including regressive evolution. In particular, we analyzed microsatellite and mitochondrial data of seven cave and seven surface populations from Mexico to test whether the evolution of the cave fish represents a case of parallel evolution. Our data revealed that Astyanax invaded northern Mexico across the Trans-Mexican Volcanic Belt at least three times and that populations of all three invasions adapted to subterranean habitats. Significant differentiation was found between the cave and surface populations. We did not observe gene flow between the strongly eye and pigment reduced old cave populations (Sabinos, Tinaja, Pachon) and the surface fish, even when syntopically occurring like in Yerbaniz cave. Little gene flow, if any, was found between cave populations, which are variable in eye and pigmentation (Micos, Chica, Caballo Moro caves), and surface fish. This suggests that the variability is due to their more recent origin rather than to hybridization. Finally, admixture of the young Chica cave fish population with nuclear markers from older cave fish demonstrates that gene flow between populations that independently colonized caves occurs. Thus, all criteria of parallel speciation are fulfilled. Moreover, the microsatellite data provide evidence that two co-occurring groups with small sunken eyes and externally visible eyes, respectively, differentiated within the partly lightened Caballo Moro karst window cave and might represent an example for incipient sympatric speciation.

Regressive evolution of an eye pigment gene in independently evolved eyeless subterranean diving beetles

Regressive evolution, the reduction or total loss of non-functional characters, is a fairly common evolutionary phenomenon in subterranean taxa. However, the genetic basis of regressive evolution is not well understood. Here we investigate the molecular evolution of the eye pigment gene cinnabar in several independently evolved lineages of subterranean water beetles using maximum likelihood analyses. We found that in eyeless lineages cinnabar has an increased rate of sequence evolution, as well as mutations leading to frame shifts and stop codons, indicative of pseudogenes. These results are consistent with the hypothesis that regressive evolution of eyes proceeds by random mutations, in the absence of selection, that ultimately lead to the loss of gene function in protein-coding genes specific to the eye pathway.

Population genetic patterns revealed by microsatellite data challenge the mitochondrial DNA based taxonomy of Astyanax in Mexico (Characidae, Teleostei)

Astyanax has become an important model system for evolutionary studies of cave animals. We investigated correlations of population genetic patterns revealed by microsatellite data and phylogeographic patterns shown by mitochondrial DNA sequences in Mexican cave and surface fish of the genus Astyanax (Characidae, Teleostei) to improve the understanding of the colonization history of this neotropical fish in Central and North America and to assess a recent taxonomic classification. The distribution of nuclear genotypes is not congruent with that of the mitochondrial clades. Admixture analyses suggest there has been nuclear gene flow between populations defined by different mitochondrial clades. The microsatellite data indicate that there was mitochondrial capture of a cave population from adjacent populations. Furthermore, gene flow also occurred between populations belonging to different nuclear genotypic clusters. This indicates that neither the nuclear genotypic clusters nor the mitochondrial clades represent independent evolutionary units, although the mitochondrial divergences are high and in a range usually characteristic for different fish species. This conclusion is supported by the presence of morphologically intermediate forms. Our analyses show that the Trans-Mexican Volcanic Belt limited gene flow, but has been crossed by Astyanax several times. In Yucatán, where obvious geographic barriers are missing, the incongruence between the distribution of nuclear and mitochondrial markers reflects random colonization events caused by inundations or marine transgressions resulting in random phylogeographic breaks. Thus, conclusions about the phylogeographic history and even more about the delimitation of species should not be based on single genetic markers.

Description of a new species from Laguna Chichancanab, Yucatan, Mexico: Cyprinodon suavium (Pisces: Cyprinodontidae)

Cyprinodon suavium is a new species that belongs to the endemic species flock from Laguna Chichancanab, Yucatan, Mexico, which is proposed to have evolved by sympatric speciation in the lake during the last 8000 years. C. suaviumis distinguished from all other known Cyprinodon species by a flattened and concave inter- and postorbital skull roof and a terminal mouth with distinctive thickened lips. The short gut length and dietary items found in the gut indicate that C. suavium is one of the carnivorous members of the flock.

Regressive and Constructive Traits in Astyanax Surface and Cave Fish

The surface and cave forms of the characid Astyanax have become a model system of outstanding importance in evolutionary research. It is extraordinary, because this diurnal day-active fish only exhibits minimal preadaptive traits allowing life in continuous darkness. In Astyanax, transition into the new environment took place abruptly and without a phase of gradual adaptation, as is usual when night-active, so-called troglophile surface forms start colonizing caves. Because of the specific characteristic of the surface fish, a large discrepancy and a large number of different traits have evolved in the cave form. The comparatively quick change into a drastically different environment is probably one of the reasons why surface and cave fish are still interfertile and can be genetically analyzed and phenotypically compared. Furthermore, cave colonization has been performed, at least in part, in parallel at different geographically separate locations and at different times. As a result, variously advanced stages of phenotypic evolution and their genetic bases can be studied.

Complexity of Interrelationship Between Astyanax Cave and Surface Fish

The haplotype distribution of the Astyanax surface fish and the origin of its cave forms is strongly influenced by Pleistocene climatic change, due to which large-scale extinction and recolonization by the surface fish took place. Because of this history and the persisting interfertility, the relationship between the Astyanax strongly eye- and pigment-reduced (SEP) and variably eye- and pigment-reduced (VEP) cave populations as well as the recent surface populations is very complex. Although based on nuclear genes clustering with the phylogenetically old SEP, the Pachon and Yerbaniz cave populations are exceptional and show close relationship to both the recent surface and the phylogenetically young VEP cave fish due to mitochondrial capture. Furthermore, the variability of eye size observed in the VEP, especially in the Chica cave population, was often explained by hybridization. This contrasts with the finding that no gene flow exists in the SEP Yerbaniz and VEP Micos cave fish populations, where cave and starved surface fish co-occur. This is explained by the surface fish being washed into the caves and not being able to compete with the well adapted SEP or VEP cave fish in darkness. These observations are in accordance with Gause’s Law which predicts that two ecomorphs competing for the same resource cannot coexist in the same niche. In the cave habitat the cave fish are at an advantage, which will finally lead to the extinction of the surface specimens competing for food. This process is reinforced in the Astyanax caves every year by periodic low food supply outside the rainy seasons. Bottlenecking obviously regularly occurs, which is demonstrated by the low variability of several genetic markers compared with the surface fish. However, mitochondrial introgression in the SEP cave populations like Pachon and Yerbaniz has nonetheless occurred. It is suggested that the introgressed modern haplotypes in the SEP Pachon and Yerbaniz cave fish derive from VEP cave populations, which, like the recent VEP Micos cave fish, were well adapted to cave life and therefore able to co-exist and also to hybridize with the SEP cave populations after the cave systems merged due to karst erosion. The recent surface fish on the one hand and the SEP and VEP cave populations on the other no longer hybridize in nature. In accordance with the Biological Species Concept, the SEP and VEP cave populations therefore can be denominated as a species on their own. The cave fish speciation process provides one of the rare examples of parallel speciation.

Surface and Cave Populations of Mexican Astyanax

The Neotropic large-eyed and well-pigmented diurnal characid fish Astyanax has developed a series of cave populations in Northeastern Mexico. These divide morphologically into a group of strongly eye- and pigment-reduced (SEP) cave populations and another one characterized by variable eye size and pigmentation (VEP cave populations). Molecular and biogeographic data imply that they derive from the Neotropic Astyanax surface fish, which were able to invade North America up to the Rio Grande drainage after the closure of the Central American land bridge. Its recent distributional pattern is strongly influenced by Pleistocene climatic changes and is characterized by regional extinction and recolonization from the warmer south and/or survival in climatically buffered refuges. An example of this are the SEP cave fish populations, which according to cytochrome b analysis do not cluster with the surface fish from neighboring rivers and creeks but with fish from a remote location about 500 km away in the Central Mexican Plateau. In line with this, they do not group with either the VEP cave fish or with surface fish from the cave area, and based on microsatellites and SNP studies, they exhibit relation to populations from southern Mexico and Belize. The SEP cave fish and some relic surface fish populations from isolated locations all over Mexico derive from the oldest invasion. In contrast, based on cytochrome b studies, the VEP cave populations cluster with the recent surface fish from the cave area, which is widespread in Northern Mexico. The VEP cave populations derive from a more recent invasion of surface fish into Northern Mexico. In particular, the differing degree of eye reduction between SEP and VEP cave fish reflects the different times of cave entry. Cave colonization in VEP and SEP cave populations took place in parallel and resulted in multiple convergent evolutions.

The Role of Rudimentation in Evolution

Processes of regression and rudimentation are deeply involved in the evolution of life and are as important as constructive evolution. They occur in every taxonomic group and concern morphological, behavioural, as well as physiological traits. For example, whales have reduced their hind legs and the pelvic girdle. The ratite birds have convergently abandoned the ability to fly and exhibit reduced wings and sternal carina. In addition, the delicate feather structure is broken down. In the Pacific island of Tahiti, where no insectivore bats exist, noctuid moths have lost the acoustic startle response. Even the gustatory system may selectively lose taste components (e.g. sweet in cats; bitter, sweet, and umami in penguins; or umami in the giant panda after changing their diet during evolution). However, from the view of human beings relying on sight as the dominant sense, the most bizarre and striking examples for rudimentation—often also characterized as degeneration or regression of traits—are provided by the loss of eyes and dark pigmentation in species living in the continuous absolute darkness of subterranean habitats like caves.

Mechanisms of Regressive Evolution

In the face of the neodarwinian paradigm of selection as an agent exclusively claimed to explain evolutionary processes in recent days, almost countless efforts have been made to prove that selection plays an important role in rudimentation processes in cave animals. Efforts mostly focus on the eye and very often pleiotropy is looked upon as being responsible. For example, eye reduction is claimed to antagonistically drive the improvement of taste or the lateral line sense by pleiotropy. However, this could not be confirmed by crossing analysis or by quantitative trait loci mapping. Energy savings have been suggested as another selection factor. This hypothesis implies that all cave species would have to suffer from food limitation. This attempt ignores the fact that the majority of tropical caves, and even some subtropical ones, abound in energy supply. Nonetheless, the traits, having become functionless in the respective cave species occurring in these habitats, regress. Thus, energy limitation is not able to explain regressive evolution of biologically functionless traits in general, or in particular that of the eye in cave species. In fact, independent inheritance of traits suggests that Astyanax cave fish are subjected to mosaic evolution.

Evolution in the Dark: Introduction

Evolution is predominantly understood as a progressive process, and less attention is usually paid to those traits being reduced at the same time. Since Darwin, who was the first to expound what can be called ‘Darwin’s loss’, the main agents of regression have been under dispute.