The karyotypes of all 10 species included here are broadly similar, with 2n = 26 + X0 (♂), and 26 + XX (♀). The autosomes are mainly either metacentric or submetacentric, and their RCLs range from about 11 to about 6. The X chromosome is metacentric and the largest in the nucleus, with RCL normally ranging from about 12–16. C-banding, where known, is confined to the centromere regions.

Subgenus Gyrinulus Zaitzev, 1907

G. minutus Fabricius, 1798

Published information: none. Mitotic chromosomes, arranged as a karyotype, are shown in Fig. 5a (plain, Giemsa-stained). Autosome pair 1 has a RCL of about 14.5, and the RCLs decrease fairly evenly along the karyotype to about 5.5 in pairs 7–13. Pair 10 has an obvious secondary constriction. Most of the autosomes are metacentric to submetacentric, with pairs 9 and 12 approaching subacrocentric. The X chromosome, clearly the longest in the nucleus, has a RCL of about 20 and is metacentric. This is the longest X chromosome encountered in the present study. Meiotic chromosomes (first division, diakinesis) are shown in Fig. 6a (plain, Giemsa-stained) and b (C-banded). The single X chromosome is clearly recognisable, as are the centromeric C-bands on all the chromosomes. Although the unpaired X chromosome appears less condensed than the autosomal bivalents, its length is in good agreement with that shown in the karyotype obtained from mitotic chromosomes (Fig. 5a). One autosomal bivalent is missing from this preparation.

Figure 5.

Mitotic chromosomes of Gyrinidae, arranged as karyotypes. a G. minutus, ♂, Isle of Lewis, testis, plain (Giemsa stained) b G. caspius, ♂, Lydd, mid-gut, plain c–e G. paykulli, ♂, Catfield Fen, mid-gut c plain d partially C-banded, still showing chromosome morphology, e the same nucleus fully C-banded, much chromosome morphology lost f, g G. distinctus fairmairei, ♂, Kuwait, testis, plain h, i G. marinus, ♂, Lydd, testis h plain i the same nucleus C-banded j, k G. opacus, Sweden, mid-gut, plain j ♂ k ♀ l, m G. opacus, Greenland, plain l ♂, mid-gut m ♀, ovary n, o G. natator, ♂, Lough Briskeen, mid-gut n plain, o the same nucleus C-banded p–s G. substriatus, ♂, testis, plain p, q Kennington r, s Isle of Lewis t, u G. suffriani, ♂, Catfield Fen, mid-gut, Giemsa stained t plain, u with spontaneous C-type banding v, w G. urinator, ♂, Tilford, testis v plain w the same nucleus C-banded x, y Orectochilus villosus, ♂, Stonesfield, mid-gut x plain, y the same nucleus C-banded. The scale line to the right of the autosome rows of u, v represents 5 μm. The vertical lines on the left-hand side link karyotypes of the same species.

Figure 6.

Meiosis, first metaphase from testis. a, b G. minutus, a plain (Giemsa stained) b the same nucleus C-banded c G. substriatus, Isle of Lewis, plain (Giemsa stained) d, e G. urinator d plain (Giemsa stained), e the same nucleus C-banded. The scale line represents 5 μm.

Saxod and Tetart (1967, 1968) used only testis as a source of chromosome preparations. Their papers give photographs of chromosomes of five Gyrinus species. They presented karyotypes of four of these, and in these cases prepared drawings of the chromosomes in situ from the relevant photographs. These drawings were then used to pair up the chromosomes to prepare karyotypes. Before giving a species by species comparison of the data, one general point should be noted: Saxod and Tetart’s illustrations consistently show secondary constrictions on various chromosomes which are more numerous and more distinct than in our material. This appears to be associated with the spreading of partially fixed chromosomes in their technique as against living material in ours.

G. caspius. Fig. 7a shows the karyotype presented here while Saxod and Tetart’s photograph and drawing are shown arranged in accordance with our karyotype in Fig. 7b, c and their drawing according to their arrangement is shown in Fig. 7d. The karyotypes shown in Fig. 7b, c show an obvious mismatch in pair 12, also present, though less marked, in Fig. 7a, while the arrangement in Fig. 7d shows mismatches in pairs 1, 4 and 11. The cause of these apparent mismatches is either different degrees of condensation between the replicates of a chromosome pair, if the pairing is correct, or a mispairing if it is not. Different degrees of condensation between the two replicates of a chromosome pair are a frequent occurrence which has to be contended with when assembling karyotypes. In the present case the appearance of the X chromosome in Fig. 7a is strikingly different from the two replicates of autosome pair 1 and is sufficient to show that this attribution is correct, especially as it is consistent with data obtained from other species where the unpaired X chromosome is shown in diakinesis of first division of meiosis.

Figure 7.

Mitotic chromosomes of Gyrinus spp, arranged as karyotypes, to compare the present results with those of Saxod and Tetart (1967) and Tetart and Saxod (1968). a–d G. caspius, a present material (Fig. 5b) b Saxod, Tetart, photograph (Plate 1B) c idem, drawing (Fig. 2), arranged as Fig. 5b, d the same drawing as arranged by Saxod, Tetart e–h G. paykulli, e present material (Fig. 5c) fTetart and Saxod (1968), photograph (Plate1A) g idem, drawing, (Fig. 1), arranged as e, h the same drawing as arranged by Tetart, Saxod i, j G. distinctus fairmairei, present material (Fig. 5f, g) k–m G. distinctus distinctus from Saxod and Tetart (1967)k photograph (Plate 1A) l idem, drawing (Fig. 1), arranged as the present G. d. fairmairei (i, j) m the same drawing as arranged by Saxod, Tetart n–q G. substriatus n present material (Fig. 5q) o drawing by Tetart, Saxod (Fig. 2) but with the X chromosome taken from their photograph (idem, Plate 1C), arranged as present material (n) p the same drawing as arranged by Tetart, Saxod. The partial X chromosome is placed as the right-hand replicate of chromosome 13 q karyotype prepared from Saxod, Tetart (Plate 1D) r, s G. suffriani r present material (Fig. 5t) s karyotype prepared from Saxod and Tetart 1967 (Plate 1C). The horizontal scale-line represents 5 μm. The vertical lines on the left hand side link karyotypes of the same species.

G. paykulli. Fig. 7e shows the karyotype presented here, while Tetart and Saxod’s material is shown in Fig. 7f (photograph) and g (drawing), arranged according to our interpretation, and the drawing according to their interpretation is shown as Fig. 7h. In this case our interpretation of Tetart and Saxod’s pictures shows minor mismatches in pairs 1 and 9, while their arrangement shows more serious mismatches in pairs 2 and 5, a different minor mismatch in pair 1 and the same minor mismatch in pair 9. Saxod and Tetart (1967) figure diakinesis of first division of meiosis, and this photograph is shown in Fig. 8c. The unpaired X chromosome is clearly visible and about as long as the longest bivalents. This agrees with our interpretation.

G. distinctus. Fig. 7i, j shows the karyotype presented here for var. fairmairei, while Saxod and Tetart’s material is shown to the same arrangement if Fig. 7k (photograph) and l (drawing), with the drawing as arranged by Saxod and Tetart shown in Fig. 7m. Our arrangement shows no serious mismatches between replicates of a chromosome pair, but minor mismatches in pairs 12 and 13, while Saxod and Tetart’s arrangement shows serious mismatches in pairs 1 and 2, but no mismatches in pairs 12 and 13. The other thing to note is that the detailed sequence of chromosome dimensions obtained from our material agrees very closely with those obtained when Saxod and Tetart’s data are arranged in the same way. These data give no support to placing var. fairmairei as a species separate from G. distinctus.

G. substriatus. Fig. 7n shows the karyotype presented here, while Fig. 7o shows Tetart and Saxod’s drawing arranged in the same way, and Fig. 7p shows the drawing as arranged by Tetart and Saxod. Finally, Fig. 7q shows a karyotype assembled according to our arrangement, from the photograph given by Saxod and Tetart. This species apparently gave Saxod and Tetart some difficulty. They chose not to assemble a karyotype from their 1967 photograph, perhaps because they thought that one of the chromosomes, lying at some distance from the others, might not belong to the same nucleus. The 1968 photograph they used is shown as their Plate 1 C and the drawing prepared from it as Fig. 2. The chromosomes as shown in the photograph are too condensed and clumped to allow assembly of a karyotype, but the drawing seems adequate. However, the photograph shows the X chromosome (as here interpreted) with a very large secondary constriction but in the drawing the distal part of the chromosome, beyond the constriction, is omitted. The chromosomes are all very condensed and neither arrangement of their material shows any obvious mismatches. However, G. substriatus is a species for which we have a preparation showing the unpaired X chromosome at diakinesis in the first division of meiosis. This leaves no doubt that this is a large chromosome. Tetart and Saxod (1968, Plate 1 E) show a first meiotic metaphase for this species. The chromosomes are very condensed and not clear, but what appears to be the X chromosome is as long as the longest autosomal bivalents. The karyotype shown in Fig. 7q shows the same secondary constriction in the X chromosome as that shown in Fig. 7o, p, but shows the X chromosome relatively shorter than in our material (Fig. 7n), though this is less apparent when the more condensed (shorter chromosomes) Isle of Lewis material (Fig. 5r, s) is considered.. There is also an obvious mismatch in pair 1. It seems likely that the apparent difference in X chromosome length between our material and Saxod and Tetart’s results from the more condensed chromosomes in their material, with the mismatch in pair 1 in Fig. 7q resulting from uneven condensation.

G. suffriani. The karyotype presented from our material is shown in Fig. 7r and one assembled from the photograph given by Saxod and Tetart (1967) is shown in Fig. 7s. The two agree very closely. Saxod and Tetart did not give a karyotype for G. suffriani, only the chromosome number. The X chromosome is the longest in both Figures and shows a secondary constriction in the long arm of Saxod and Tetart’s material, comparable with that in G. substriatus. The secondary constriction in the short arm of chromosome 2 is apparent in both our material and that of Saxod and Tetart.

The data presented here show that the 10 species of Gyrinus discussed here all have broadly similar karyotypes, with 13 pairs of autosomes X0 sex chromosomes, and the X chromosome the longest in the nucleus. There are often small differences between the chromosomes of different species. Thus the relative length of the X chromosome of G. minutus is about 1.5 times that of the longest autosome, a bigger difference than in any of the other species studied.

There are two species-groups among the studied species. G. caspius and paykulli are conspicuously elongate parallel-sided beetles, though with very different aedeagi. Unfortunately the chromosomal material presented here is inadequate to show interspecies differences. The second group, G. natator, substriatus and suffriani, does show some interspecies differences. In G. natator the more or less submetacentric chromosome 2 is clearly longer than metacentric chromosome 3, while in G. substriatus metacentric chromosome 2 is longer than submetacentric 3, though only slightly so. The X chromosome is only slightly longer than chromosome 1 in these species. In G. suffriani the X chromosome is more clearly longer than chromosome 1, chromosome 4 has a distinct secondary constriction in its short arm, and there is more marked decrease in length between pairs 4 and 5.

There are two cases where conspecific material from widely separated localities shows no chromosomal difference. This is true of Swedish and Greenland G. opacus, with the Greenland material belonging to the form which lacks elytral reticulation, and of Kuwaiti and French G. distinctus, with the Kuwaiti material belonging to var. fairmairei with a yellow underside as against the largely black underside of the French material.

Gyrinus urinator is the only species with acrocentric chromosomes–chromosome 12.

The chromosomes of O. villosus agree with those of Gyrinus spp. in having an X0 sex chromosome system and a relatively long X chromosome. However, they differ in having 16 pairs of autosomes as against 13 in Gyrinus, and in having the three longest pairs with heterochromatic long arms, then a fairly long pair with a largely heterochromatic longer arm, and the remaining pairs short to very short with small centromeric C-bands, ranging from metacentric to subacrocentric.