More than four decades after a young lecturer at Royal Holloway College first coaxed readable chromosome preparations out of beetle embryos, an updated atlas of Helophorus chromosomes has arrived, doubling the number of species it covers and offering one of the most detailed karyotypic surveys ever assembled for a beetle genus. The new account, published in Comparative Cytogenetics, documents chromosome preparations for 62 of the roughly 170 Palaearctic species of Helophorus, a genus of water scavenger beetles that has long frustrated taxonomists because many of its species look almost identical from the outside.
The atlas is the work of Robert Angus, who began investigating Helophorus chromosomes in 1975 after earlier attempts at chromosome preparation had ended in complete failure. The breakthrough came from an acetic acid dissociation and air-drying technique described by Crozier in 1968, which allowed well-spread, undistorted chromosomes to be prepared. Angus applied Giemsa stains to similarly prepared material, following protocols already established for mammalian chromosomes, and obtained excellent results. All of his early preparations came from developing embryos, a choice that would shape the entire research programme.
At the heart of the atlas lies a long technical struggle over banding. C-banding, which highlights blocks of highly repetitive DNA, was already well established in insects and can be produced by treating chromosomes with alkali followed by incubation in salt-sodium citrate at about 60 degrees Celsius. G-banding, the finer, more informative banding that allows chromosomes and even sections of chromosomes to be identified with great precision, was another matter entirely. Published accounts of G-banding in insects were discouraging: the few insect studies available showed only a handful of bands, or produced patterns that were ragged and difficult to interpret. The rewards, however, were potentially enormous, because fine-grained banding can demonstrate homologies between the chromosomes of different species, much as it revealed the relatedness of humans and the great apes, and of the giant panda and the brown bear.
After much experimentation, Angus found that very slight trypsin treatment of freshly prepared slides, quenched after only a few seconds at 10 degrees Celsius, could produce numerous bands on all the chromosomes. The problem was that the resulting banding was extremely fine-grained and very even, raising a fundamental question: did it reflect genuine chromosome organisation, or merely the last bits of chromosome not destroyed by the enzyme? The answer came from studies by Rønne and colleagues, who had used various reagents on in vitro cultures of human cells. When Angus treated developing Helophorus embryos in vitro with 5-fluorouridine, the same chromomere banding appeared, indicating that the banding was genuine rather than an artefact of trypsin digestion.
The fine-grained banding proved remarkably informative. A comparison of the longest autosomes of Helophorus aquaticus and H. aequalis shows a similar sequence of bands, allowing for differences in the size of their centromeric C-bands, except in the distal region of the short arm, where the two species differ in ways suggesting that these apical sections have been involved in translocations. The banding patterns appear to correspond with bands observed on pachytene chromosomes during meiosis, a feature of mammalian G-banding, which supports the interpretation that these are genuine G-bands rather than something else.
The atlas itself divides the genus into two karyotype-groups. Species of the subgenera Helophorus s. str., Gephelophorus and Eutrichelophorus have eight pairs of autosomes plus Xyp sex chromosomes, in which the very small y chromosome is attached to the X by a nucleolus or cytoplasmic vesicle, an arrangement known as the parachute-association. Species of the subgenera Empleurus, Trichohelophorus, Lihelophorus and Rhopalohelophorus have ten pairs of autosomes plus Xyp sex chromosomes. Within these broad groups, the karyotypes show considerable variation in chromosome shape, in the size and position of C-bands, and in the location of nucleolus organisers, the sites of ribosomal RNA genes that can be revealed by silver staining.
One of the most distinctive karyotypic features in the genus is a secondary constriction on one chromosome, confirmed by silver staining as the site of a nucleolus organiser, originally discovered in H. aequalis. In that species the NOR-bearing chromosome goes as pair 6 in the karyotype, and in other species of Helophorus s. str. the NOR-bearing chromosome is placed as pair 6 for ease of comparison. The atlas documents how the size and shape of this chromosome varies between species, and how ongoing DNA analysis by Martin Fikáček has revealed unexpected relationships, such as the finding that H. grandis, the first of the big-toothed group of species, is the sister-species of H. aequalis, a relationship supported by the size and shape of the NOR-bearing autosome 6.
Chromosome polymorphisms are widespread in the genus. Pericentric inversions, which flip a section of chromosome containing the centromere and change the position of the centromere itself, have been found in several species, including H. grandis, H. milleri and H. brevipalpis, in which an autosome may be either metacentric or acrocentric depending on the inversion state. Addition of extra C-banding regions has also been recorded, and small B-chromosomes, supernumerary chromosomes that are not part of the standard complement, have been found in a few species, including H. nevadensis and H. jocoteroi. In general, karyotypes have proved very useful in establishing the limits of individual species, and for groups such as H. leontis, H. dixoni and H. biltoni, which cannot be separated by their aedeagal morphology, the karyotypes leave no doubt that they are separate species.
Parthenogenesis involving triploidy has been found in two species. The first chromosomally proven case was in H. brevipalpis, where triploid females were found in the Spanish province of León, accompanied by diploids of both sexes. Triploid females have since been found in Italy, and a chromosomally proven triploid female of H. orientalis has been recorded from China. A solitary triploid embryo was also found among numerous normal diploids from egg cocoons of a French H. aequalis, an occurrence that appears to have no possibility of hybrid origin, since no other known species of Helophorus s. str. has chromosomes sufficiently similar to those of H. aequalis to produce such a convincing triploid karyotype. Whether the variations in replicate length within the triplets of these triploids result from slight random variation in chromosome condensation or from a hybrid origin of the triploids themselves remains an open question, and the appreciable variation within H. brevipalpis might indicate hitherto undetected cryptic species.
Karyotypes of experimentally produced hybrids have revealed irregularities in chromosome condensation. Hybrids between H. lapponicus and H. paraminutus, and between H. minutus and H. paraminutus, show serious mismatches in the lengths of the replicates of several chromosomes, a result that was not at all what was expected when the crosses were originally undertaken with a view to obtaining karyotypes in which chromosome condensation was completely synchronised. Some of these hybrids were reared through to adulthood and were apparently able to produce functional meiosis with no failures of chromosome pairing. The updated atlas, with its steady accumulation of karyotypes, its documentation of polymorphisms, B-chromosomes and parthenogenesis, and its demonstration that fine-grained banding reflects genuine chromosome organisation, stands as a testament to what patient, careful cytogenetics can achieve, and it provides a foundation for anyone seeking to understand the evolutionary genetics of this remarkable beetle genus.
Subject of Research: An updated Atlas of Helophorus chromosomes
Article Title: An updated Atlas of Helophorus chromosomes
Article References: Angus, R. B. (2023). An updated Atlas of Helophorus chromosomes. Comparative Cytogenetics, 17, 295-326. https://doi.org/10.3897/compcytogen.17.112831
Image Credits: AI Generated
DOI: 10.3897/compcytogen.17.112831
Keywords: updated, Atlas, Helophorus, chromosomes, scientific research, peer-reviewed research
