Different behaviour of C-banded peri-centromeric heterochromatin between sex chromosomes and autosomes in Polyphagan beetles

Abstract

Heterochromatin variation was studied after C-banding of male karyotypes with a XY sex formula from 224 species belonging to most of the main families of Coleoptera. The karyotypes were classified in relation with the ratio heterochromatin/euchromatin total amounts and the amounts of heterochromatin on autosomes and gonosomes were compared. The C-banded karyotypes of 19 species, representing characteristic profiles are presented. This analysis shows that there is a strong tendency for the homogenization of the size of the peri-centromeric C-banded heterochromatin on autosomes. The amount of heterochromatin on the X roughly follows the variations of autosomes. At contrast, the C-banded heterochromatin of the Y, most frequently absent or very small and rarely amplified, looks quite independent from that of other chromosomes. We conclude that the Xs and autosomes, but not the Y, possibly share some, but not all mechanisms of heterochromatin amplification/reduction. The theoretical models of heterochromatin expansion are discussed in the light of these data.

Keywords: Coleoptera; Polyphaga; heterochromatin; karyotypes; sex chromosomes; variation.

Figure 1.

C-banded male karyotypes. AAdaliabipunctataBAsidajurineiCMorimusfunereusDCyclocephalapicipes. The autosomes and the X chromosomes have similar amounts of C-banded heterochromatin, but the Y chromosomes remain unstained.

Figure 2.

C-banded male karyotypes. AAmphimallonsolstitialeBDorcadionetruscumCLiliocerisliliDStrategussyphax. In each karyotype, all centromere regions are similarly C-banded, but that of the Y is more variable.

Figure 3.

C-banded males karyotypes. AUlomaretusaBLucanuscervusCDisonychalatifronsDMelolonthahippocastani. The level of heterochromatin amplification is often similar in the X and autosomes (A, B, D). The amplification may also be scattered, as in C, but it rarely involves the Y chromosome.

Figure 4.

C-banded male karyotypes. ALeucothyreusnolletiBPropomacrusdavidiCLamprimaadolphinaeDScarabaeusvariolosus. Large heterochromatin amplification can involve the X alone (A, B, C) and more rarely the Y (D).

Figure 5.

C-banded male karyotypes. A, BMelolonthamelolonthaC, DMacraspistristis. At contrast with the high variability of autosomes, there is a remarquable stability of the amount of C-banded heterochromatin on the X (average in A, B and amplified in C, D).

Figure 6.

Criocerisasparagi. A C-banded male karyotype displaying a large heterochromatin amplification in all chromosomes but the Y. B Incorporation of BrdU during late S-phase in a female cell: all heterochromatin is homogeneously late replicating (orange staining). The distal fragments of all chromosomes are early replicating (green), which indirectly indicates that there is no Lyonisation of one X. C C-banding of 3 spermatocytes (a, b, c) at pachynema : autosomal bivalents are at contact and form rosettes after heterochromatin fusion. The sex bivalent is always separated. D Q-banded male karyotype: heterochromatin displays at least 3 levels of fluorescence.

Figure 7.

QM-staining of Macraspistristis cells. A Spermatogonium B, D 9,X and 9,Y spermatocytes II C 8+Xyp spermatocyte I at diakinesis/metaphase. Heterochromatin, in particular that of the X, displays very different levels of fluorescence. e= euchromatin, h=heterochromatin.