Helophorus brevipalpis is one of the most widespread and abundant water scavenger beetles in Europe, famous among entomologists for its spectacular mid- to late-summer swarming behavior, when enormous numbers of adults may descend on shiny surfaces such as car bodies and glass windows. Earlier ecological work in the Oxford area of England by Fernando in 1958 established that this mass dispersal phase is associated with newly emerged adults whose ovaries are still in early stages of development. Similar swarming was documented in central Sweden by Landin in 1958. None of this classic research gave any hint that parthenogenesis, reproduction without fertilization by males, might be involved in the species’ life cycle.

The first clue came from an unexpected direction. In 1971, a population consisting entirely of females was discovered in Logan Canyon, Utah, representing the only genuine North American record of the species and suggesting it had been introduced there. The first triploid parthenogenetic populations in Europe were identified from the Provincia de León in Spain, where they occurred alongside normal bisexual diploid populations. A subsequent discovery established triploid females in the Po valley of Italy. The newly reported French material from the Camargue and the Hérault region now represents a third distinct area inhabited by these chromosome-tripled females.

The living material studied comprised twelve triploid females collected from a puddle near the Étang de Vaccarès in the Camargue, Bouches du Rhône, France, one triploid female from the Salagou River in Hérault, and four diploid specimens, three females and one male, from a puddle by Setley Pond in the New Forest, Hampshire, England. Older preserved material was also reworked, including a diploid male from Crete, Greece, and a diploid male from the Golan region of Israel. Chromosome preparations were made from mid-gut and spermatogonial cells, the latter complicated by the fact that adjacent cells in the spermatogonium synchronise their positions in the cell cycle and the membranes between adjacent cells may be weak or even absent.

A key technical achievement of the new work is the successful application of C-banding, a staining technique that highlights regions of constitutive heterochromatin, to both triploid and diploid material. The method involved treatment for seven to eight minutes in a saturated solution of barium hydroxide at room temperature, followed by incubation in a salt-sodium citrate solution at 55 degrees Celsius. Earlier attempts using longer treatment times destroyed all stainability of the chromosomes entirely, while shorter treatments produced no banding at all. Banding was eventually achieved through a modified protocol involving a further two minutes in barium hydroxide followed by a further forty-five minutes in the citrate solution.

The karyotypes of the French triploids appear to be identical with those of Spanish material recorded in 1992, but show minor differences from Italian triploid material described more recently. In both French and Spanish triploids, the sequence of autosomes arranged in order of decreasing length shows the same pattern of centromere positions. Autosomes one, three, five and seven are clear metacentrics, while six, eight and ten are borderline metacentric or submetacentric. Autosome two is subacrocentric and autosomes four and nine are acrocentric. The X chromosome is borderline submetacentric or subacrocentric and about the same length as autosomes six or seven. C-banding in the French triploids shows bands confined to the centromere regions, present in all the autosomes and the X chromosomes. The Italian triploids closely resemble the Spanish and French material except that there is no subacrocentric in position three, which is clearly metacentric instead.

Hitherto, C-banding had not been demonstrated in diploid Helophorus brevipalpis. The banding reported here comes from Southern English material in Hampshire’s New Forest. Moderate to weak centromeric C-bands are present on all the autosomes as well as on the X chromosome. The variation in the apparent strength of these bands is almost certainly associated with the difficulty experienced in obtaining them. The diploid karyotypes show some slight variation across the species’ range. The Spanish specimen has autosome two subacrocentric, as in the Spanish and French triploids, but in all other diploids this chromosome is metacentric, while pair three, metacentric in the Spanish specimen, is clearly submetacentric in the others. The Cretan specimen has autosome six clearly metacentric as against submetacentric or subacrocentric in the other populations.

Triploid Helophorus brevipalpis appear to result in often abundant females-only populations. The superabundance of females in the Camargue population immediately aroused suspicion, but the triploidy of the single Hérault specimen came as a total surprise. The Spanish triploids occur at the edge of the species’ range, where H. brevipalpis appears to be absent from neighboring Galicia. The situation in Italy is less clear, with populations in the Po valley and adjacent territory composed of triploid females, but males occurring further south. The parthenogenetic population in Utah, of unknown ploidy, may be the result of a female resorting to parthenogenesis in the absence of males, a phenomenon also observed in female Komodo Dragons in zoos.

An earlier morphological study from 1988 sought to make sense of the considerable variation, both somatic and in male genitalia, across the species’ range and described a new subspecies, H. brevipalpis levantinus, from disparate areas in Turkey, Lebanon and Iran. Discriminant functions analysis grouped all these populations together and separate from other populations involved. A dendrogram showing the relative distances between population centroids reveals the three levantinus groups well clear of all the others, with the exception of one population east of Lake Van regarded as a possible intermediate. This fairly clear separation of the subspecies means that consideration of the material in the current paper may be confined to the nominate subspecies brevipalpis. Examination of the dendrogram shows material from Northwest Europe, Italy and Greece to come out as a close group, along with material from Perm in the Urals, and with material from Turkey and Israel not far away. Material from Spain, Avignon and Odessa, as well as that from northwest Iran and Alpine Georgia, also comes together as a close group. This somewhat surprising juxtaposition of geographically distant populations is a warning to expect surprises when interpreting the species’ population structure.

The morphological work discussed is now almost forty years old, and the obvious question is what modern DNA analysis might reveal. DNA sampling of multiple populations of Helophorus brevipalpis would be highly desirable and may provide answers to the puzzles of chromosomal variation and population history. However, some caution is needed, as the separation of various populations may have been too recent to show clear genetic differences. The discovery of triploid females in Provence adds another piece to this intricate puzzle, demonstrating that parthenogenetic lineages of this common beetle are more widespread across southern Europe than previously recognised, and highlighting how much remains to be learned about the chromosomal evolution of one of Europe’s most familiar water beetles.

Subject of Research: Triploid parthenogenetic populations and chromosomal variation in the water beetle Helophorus brevipalpis

Article Title: Triploid female Helophorus brevipalpis Bedel, 1881 in Provence, France, with additional data on C-banding in both triploid and diploid material, and discussion of chromosomal variation in H. brevipalpis

Article References: Angus, R. B. (2025). Triploid female Helophorus brevipalpis Bedel, 1881 in Provence, France, with additional data on C-banding in both triploid and diploid material, and discussion of chromosomal variation in H. brevipalpis. Comparative Cytogenetics, 19, 125-131. https://doi.org/10.3897/compcytogen.19.162685

Image Credits: AI Generated

DOI: 10.3897/compcytogen.19.162685

Keywords: Helophorus brevipalpis, triploidy, parthenogenesis, C-banding, karyotype, chromosomes, Coleoptera, Provence, Camargue, cytogenetics, water beetle, chromosomal variation