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FIGURE 1 Mitotic and meiotic holocentric chromosomes in an earwig, Labidura truncata. Orcein-stained squash preparations, B, L, M-P colchicine-treated. (A) Spermatogonial division in prophase with the Y at bottom left and the X to the right, both more condensed than the autosomes.

(B) Spermatogonial metaphase with the small Y chromosome obvious.

(C) Leptotene, with the sex chromosomes at the top very condensed and the heterochromatic ends of some autosomes also condensed. Two nucleoli are visible, one at 11 o'clock and the other at 5 o'clock. (D) Zygotene—pachytene with the heterochromatic ends of the autosomes more obvious. (E) Diplotene displaying the four autosomal bivalents and the condensed sex chromosomes separately. (F) Diakinesis, one autosomal bivalent showing a chiasmata that is quite interstitial. (G, H) First metaphases with the larger X seem to be paired with the smaller Y. First anaphase with the neocentromere actively moving the chromosomes apart. (J, K) Second metaphases; J shows the X dyad, K shows the smaller Y dyad. (L—P) Female mitotic chromosomes, late and early prophase in L and N, respectively; M—P show metaphases, with O and P showing secondary constrictions. The primary constrictions of fixed centromeres do not show, and uninterrupted chromatids, characteristic of holocentric chromosomes, are particularly obvious in M. [From Giles, E. T., and Webb, G. C. (1973). The systematics and karyotype of Labidma Truncata Kirby, 1903 (Dermoptera: Labiduridae). J. Aust. Entomol. Soc. 11, Plate 1, with permission.]

about 20% of all animal species. This mechanism has allowed one species of Australian ant, Myrmecia croslandi, to achieve the lowest possible chromosome number, n = 1, in the parthenogenically derived male.

Chromosomal imprinting has not been demonstrated in insects, so gametes from both sexes are not necessarily required. Indeed, accidental development of unfertilized eggs (thelytoky) can form a parthenogenetic insect if sufficient double-haploid cells arise in critical tissues in the n/2n mosaic.

Sex chromosomes are usually involved in sex determination in insects, but by a variety of genetic mechanisms. The male is usually the heterogametic sex in insects, the exceptions being the orders Lepidoptera (butterflies and moths) and Trichoptera, in which the females are heterogametic. The mammalian system of having genes determining the sex and other male functions on the Y chromosome almost certainly does not occur in insects. In the earwigs (Dermaptera), male determination by the presence of a Y chromosome seemed to be the rule, until XO/XX mechanisms were found in two species.

As in other animals, the insect heterogametic male has half the number of X chromosomes as the female; most commonly the sexes are XO male and XX female, but multiple X-chromosome systems frequently occur. Fusions of autosomes to the X chromosome can cause the formation of XY/XX and further fusions to form X1X2Y/X1X1X2X2 systems. X1X2Y males are almost the rule in the mantids (Mantodea). In the most of the Hymenoptera, sex is determined by the diploid females being heterozygous, and the haploid males hemizygous, for multiple alleles at a single genetic locus on one chromosome; that might still be regarded as an X chromosome.

The karyotype is the set of chromosomes, both autosomes and sex chromosomes, in an organism. The karyotype found in 90% of the large family of short-horned grasshoppers, Acrididae, is usually given as 2n6 = 23 [22 + X (or XO)], the female karyotype, 23 (22 + XX), being usually inferred from the male. Some authors carefully confirm diploidy and subdifferentiate the autosomes and the sex chromosomes, for example, for the earwigs Chaetospania brunneri 2n 6 = 31 (13AA + X1X2X3X4Y) and Nala lividipes, with one pair of autosomes being exceptionally long, 2n6 = 34 (ALAL + 15AA + XY). Since in insects, the karyotypic nomenclature is variable and somewhat confusing, it would seem preferable to adopt the simple karyotypic nomenclature used for mammals [e.g., human: 2n6 = 46,XY].

Monocentric chromosomes are the norm in most insect orders (Fig. 2), with the single centromere characterized by a primary constriction, a structure seen in many other animals and in plants. After replication of the DNA and other chromosomal constituents during interphase, the chromosomes at metaphase show two identical chromatids. Following the discovery that each chromatid must be terminated by a telomere, geneticists concluded that it is highly probable that a chromosome must always have two arms, one on each side of the centromere. If these arms are of appreciable length, the chromosomes are called metacentric (arms of about equal length), or submetacentric (arms of unequal length). If one of the arms is very short, perhaps invisible under normal microscopy, the chromosome is said to be acrocentric. It is now widely accepted that a chromosome cannot normally be telocentric (terminated by a centromere). The sequence of nucleotides repeated many times to make up the DNA of the telomeres of most insects is TTAGG, but it is not universal.

Holocentric chromosomes occur in the insect orders Heteroptera, Dermaptera (Fig. 1), Mallophaga, Anoplura, and Lepidoptera. The centromeres are elongated across much of the length of the chromosomes, although not usually extending to the telomeres. During mitotic anaphase, the spindle fibers pull equally on most of the length of the chromosome so that only the distal ends can be seen to be

FIGURE 2 Monocentric chromosomes of the locust Chortoicetes terminifera, mostly acrocentric with some of the smaller ones submetacentric. (a) With one B chromosome, which is distinctively G-banded by a trypsin treatment that has produced comparatively minor effects in the A chromosomes. (b) with two B chromosomes showing positive C-banding for most of their length. The A chromosomes mostly have small centromeric C bands, but they show variable interstitial and distal C-banded segments.

FIGURE 2 Monocentric chromosomes of the locust Chortoicetes terminifera, mostly acrocentric with some of the smaller ones submetacentric. (a) With one B chromosome, which is distinctively G-banded by a trypsin treatment that has produced comparatively minor effects in the A chromosomes. (b) with two B chromosomes showing positive C-banding for most of their length. The A chromosomes mostly have small centromeric C bands, but they show variable interstitial and distal C-banded segments.

trailing. Operation of an elongate centromere during first meiosis would break the crossovers, or chiasmata, which have formed between the the paired chromosomes. Apparently to preserve this chromosomal bivalent, the holocentric chromosomes develop neocentric activity at one telomeric end only. The neocentromeres behave like those of monocentric chromosomes, and they persist through second metaphase of meiosis (Fig. 1). Broken holocentric chromosomes seem to be able to retain attachment to the spindle fibers: in earwigs, each piece of a broken chromosome forms a bivalent with a neocentromere during meiosis. Breakage of holocentric chromosomes probably also explains the wide range of chromosome numbers seen in butterflies, from 2n = 14 to 2n = 446, and the extreme of 2n = 4 to 2n = 192 found by Cook in a single genus of scale insect, Apiomorpha. Breakage probably also accounts for the common finding of multiple X chromosomes in insects with holocentric chromosomes. The addition of telomeres to the broken ends of holocentric chromosomes might be a function of the complex enzyme telomerase.

Polytene chromosomes are large chromosomes formed by the repeated replication, without intervening division, of chromatids that remain uncondensed as in interphase (Fig. 3). Polytene chromosomes often contain thousands of chromatid strands, and the homologous chromosomes are usually closely somatically paired, so that inversions in them are accommo-

FIGURE 3 Polytene chromosomes in the salivary glands of the larvae of two species of chironomid midge. Orcein-stained squash preparations. (a) From the North American species Chironomus decorus, species b; (b) From the Australian species C. oppositus. For both species, labels A—F indicate arms of metacentric chromosomes, with arrowheads indicating the centromeres. The acrocentric chromosome G shows some breakdown of somatic pairing at the distal end in both species. Chromosomes AB and EF in C. decorus b have undergone whole-arm exchanges to form AE and BF chromosomes in C. oppositus. N and BR indicate nucleoli and Balbiani rings, respectively. Loop pairing, resulting from heterozygosity for paracentric inversions, can be seen in arms D and F in C. decorus b and in arm D in C. oppositus (Images kindly supplied by Dr. Jon Martin, University of Melbourne.)

FIGURE 3 Polytene chromosomes in the salivary glands of the larvae of two species of chironomid midge. Orcein-stained squash preparations. (a) From the North American species Chironomus decorus, species b; (b) From the Australian species C. oppositus. For both species, labels A—F indicate arms of metacentric chromosomes, with arrowheads indicating the centromeres. The acrocentric chromosome G shows some breakdown of somatic pairing at the distal end in both species. Chromosomes AB and EF in C. decorus b have undergone whole-arm exchanges to form AE and BF chromosomes in C. oppositus. N and BR indicate nucleoli and Balbiani rings, respectively. Loop pairing, resulting from heterozygosity for paracentric inversions, can be seen in arms D and F in C. decorus b and in arm D in C. oppositus (Images kindly supplied by Dr. Jon Martin, University of Melbourne.)

dated by the formation of loops. Transcription of ribonucleic acid (RNA) from the DNA is accomplished at expanded regions called Balbiani rings, (BR in Fig. 3B), and the attachments of the polytene chromosomes to the nucleoli (N in Fig. 3B) by the nucleolar organizing regions are obvious. Polytene chromosomes have been most famously studied in the salivary glands and other glandular tissues in insects of the order Diptera, particularly in the fruit fly, Drosophila melanogaster. They display a large number of bands without any special staining, and the detail revealed is most useful for the localization of DNA sequences of various types, including single gene probes.

Supernumary or B chromosomes occur occasionally in insects of most orders. B chromosomes, when present, are in addition to the always present A chromosomes. Certainly the most variable and spectacular B chromosomes ever seen were found in the Australian plague locust, Chortoicetes terminifera

(Fig. 2). These B chromosomes display over 20 different banding patterns after treatment with trypsin, and this treatment allowed the harmless identification of carriers of B chromosomes using interphase cells in the hemolymph, thus facilitating breeding experiments. These breeding experiments showed that single B chromosomes in males of C. terminifera were distributed into the sperm with a 50% frequency, but in females single B chromosomes were driven into the egg with a frequency of 80%. This meiotic drive in females should have ensured that every individual in the population carried a B chromosome. Since, however, they were found in only 10% of individuals, the B chromosomes must have been lowering the fitness of carriers. The situation supported a "parasitic" mechanism for the maintenance of B chromosomes in the population.

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