Exydrus, BROUN, 1886

Angus, Robert B, Sadílek, David, Shaarawi, Fatma, Dollimore, Hayley, Liu, Hsing-Che, Seidel, Matthias, Sýkora, Vít & Fikáček, Martin, 2021, Karyotypes of water scavenger beetles (Coleoptera: Hydrophilidae): new data and review of published records, Zoological Journal of the Linnean Society 192 (3), pp. 958-958 : 958-

GENUS EXYDRUS BROUN, 1886

( FIG. 13A, D, I )

Material examined: Exydrus gibbosus Broun, 1886: 1 female (NMPC): New Zealand: Wellington ( WL) , Tararua Range, 1 km W of Titurea Dam, start of Greens Rd. , 40.4295°S 175.66064°E, 145 m, 26.xi.2016, fragment of broadleaf forest with sparse understory with ferns and Pandanus Parkinson : sifting, M. Fikáček & M. Seidel lgt. ( MM31 ) .

Karyotype: 2 n = 30 (♀). The only nucleus obtained has 29 chromosomes and the karyogram shows a serious size mismatch in pair 3 so there should be at least 30 chromosomes in the karyotype, presumably 14 pairs of autosomes plus the sex chromosomes. The smallest chromosome appears heavy and almost single-stranded; however, this is almost certainly because the two chromatids of a metacentric are lying on top of each other. This is frequent with small chromosomes. There is gradual decrease in length along the karyotype, with no pair strikingly longer than the others. The smallest pair is about a third of the length of the longest pair. Most of the chromosomes are metacentric or submetacentric; however, pairs 6–9 are subacrocentric.

Figures

Figure 13. Karyotypes of the Cylominae, without treatment. A, D, Exydrus gibbosus, mitosis, midgut. B–C, E, Tormissus magnulus, mitosis, midgut. F–H, Rygmodus modestus (F–G, meiotic metaphase I, testes; H, mitotic karyotype, midgut). Habitus figures: (I) Exydrus gibbosus; (J) Rygmodus modestus, from Minoshima et al. (2018).