Horst Wilkens

Genetic analysis of cavefish reveals molecular convergence in the evolution of albinism

The genetic basis of vertebrate morphological evolution has traditionally been very difficult to examine in naturally occurring populations. Here we describe the generation of a genome-wide linkage map to allow quantitative trait analysis of evolutionarily derived morphologies in the Mexican cave tetra, a species that has, in a series of independent caves, repeatedly evolved specialized characteristics adapted to a unique and well-studied ecological environment. We focused on the trait of albinism and discovered that it is linked to Oca2, a known pigmentation gene, in two cave populations. We found different deletions in Oca2 in each population and, using a cell-based assay, showed that both cause loss of function of the corresponding protein, OCA2. Thus, the two cave populations evolved albinism independently, through similar mutational events.

Evolution and Genetics of Epigean and Cave Astyanax fasciatus (Characidae, Pisces)

Notable progress in the biological sciences is occasionally made possible by the discovery of a species that exhibits a specialized way of life, but nevertheless will thrive and breed in the laboratory. In the study of cavernicolous animals, the Mexican characid fish Astyanax fasciatus has come to play a role equivalent to that of the fruit fly, Drosophila, in genetics.

Marine lava cave fauna: composition, biogeography, and origins.

An assemblage of endemic cavernicolous marine invertebrates, including taxa found on both sides of the Atlantic Ocean of great phylogenetic age or with affinities to deep sea organisms, inhabits the Jameos del Agua cave, a sea waterflooded Holocene lava tube cave on Lanzarote in the Canary Islands. This marine cave contains both relicts from Tethyan times, such as an apparently new crustacean family belonging to what had been the monotypic class Remipedia, and relicts of groups that are now common only in the deep sea as well as species that occur outside the cave.

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.

Transcription of the opsin gene in degenerate eyes of cave-dwelling Astyanax fasciatus (Teleostei, Characidae) and of its conspecific epigean ancestor during early ontogeny

The early morphogenesis of the degenerate eyes of the Mexican cave fish Astyanax fasciatus and of its conspecific epigean ancestor has been studied comparatively using light- and electron-microscopical techniques; the transcription of the opsin gene has been analysed during early ontogeny in both populations by in situ hybridization. The opsin protein is an integral component of the disk membranes of the photoreceptor cells. In epigean specimens, its expression is correlated with the beginning of outer segment formation on the third day of development. Morphogenesis of the cave fish eye is similar to that of the epigean eye until end of the third day. However, eye growth and morphogenesis of the retina are delayed and sporadic cell death occurs in all retinal layers at the beginning of the second day. Retinal cytodifferentiation breaks off at the point of outer segment formation. Cave specimens are not able to develop regular outer segments at any stage, but the opsin gene is nevertheless expressed in the outer nuclear layer of the developing retina for a limited period of time. On the basis of the comparative morphology and transcriptional studies of epigean and cave specimens, it is suggested that the eye regression of cave fish is primarily the result of mutations of developmental control genes and not of structural genes.

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.

Convergent adaptations to cave life in the Rhamdia laticauda catfish group (Pimelodidae, Teleostei)

The nocturnal Mexican catfish Rhamdia laticauda (Pimelodidae, Teleostei) is the surface sister species of a number of cave species. Comparison between two of them, R. zongolicensis and R. reddelli, reveals an intermediate state of reduction of the biologically functionless eyes, melanin pigmentation and the negative phototactic behavior. The surface species is perfectly adapted to life in darkness. Therefore only few constructive adaptations are developed in the cave species. For survival under conditions of low food supply in the caves the barbels are elongated to improve the senses of taste and touch and more fat can be deposited in the cave fish tissue. Due to convergent evolution the two cave species are morphologically much alike and show only minor diagnostic meristic differences. From geological data it can be concluded that the two cave species started troglobitic evolution at the end of Pleistocene.

POPULATION BIOLOGY AND LARVAE OF THE ANCHIALINE CRAB MUNIDOPSIS POLYMORPHA (GALATHEIDAE) FROM LANZAROTE (CANARY ISLANDS)

The galatheid Munidopsis polymorpha Koelbel, 1892, is found at high population density in an anchialine pool (Jameos del Agua) on Lanzarote. Ovigerous females on an average carry 14 eggs. Larval development is advanced. Only 2 zoeal stages, which are unable to undergo locomotion, precede molt to a small crab. Munidopsis polymorpha is not preyed upon in the Jameos del Agua. Population density is regulated less by available food than by the number of available hiding places for resting phases. Population density is probably increased by the fact that the highly territorial crabs periodically alternate in feeding in the same grazing areas and in hiding. The percentage ofovigerous females and the average number of eggs per female is negatively correlated with the number of specimens per area. During the years of investigation from 1976-1989, the number of ovigerous females and the average number of eggs per female have decreased considerably in parts of the pool. Consequently, the general population density has become drastically reduced. This is attributed to the construction and opening of the Jameos del Agua for tourism.

Multiple photopigments from the Mexican blind cavefish, Astyanax fasciatus: a microspectrophotometric study.

The cave-dwelling (hypogean) form of the teleost Astyanax fasciatus is blind, having only subdermal eye rudiments, but nevertheless maintains intact opsin genes. Second generation offspring of a cross between these and the normally sighted surface (epigean) form inherit opsin genes from both ancestries. A study of the expressed hypogean opsins of the hybrids, in comparison to the epigean forms, was undertaken by microspectrophotometry. The hybrid population showed considerable variation in the visual pigments of double cones, with evidence for two groups of cells with lambda(max) intermediate to those of the epigean pigments. Possible explanations for these intermediate pigments are discussed, including the hypothesis that they may represent hybrid genes similar to the genes for anomalous cone pigments in humans. Evidence was also found for ultraviolet-sensitive single cones and for an additional MWS pigment.