Chromosomal Rearrangements

Rearrangements occur in the chromosomes of insects when they occasionally break and rejoin in an irregular fashion. If any chromosomal rearrangement is maintained heterozygously in a population at a frequency greater than can be explained by recurrent chromosomal mutation, it is said to be polymorphic. There are a number of chromosomal rearrangements.

Paracentric Inversions

Paracentric inversions result when two breaks in one chromosome arm rejoin after the excised piece has inverted. These rearrangements are commonly recorded in polytene chromosomes, where the presence of them is shown by the formation of a loop allowing the homologues to be closely paired (Fig. 3). The presence of a chiasmata at meiosis within paracentrically inverted segments results in a dicentric chromosome and an acentric fragment, which cannnot be regularly transmitted. Paracentric inversions survive for long periods in many dipteran species because there is no chiasma formation in males and because the products of female meiosis are organized to ensure that a nonrecombinant for any paracentric inversion is deposited in the egg nucleus, with recombinants being placed in the unused polar bodies.

Because they have the capacity to lock up long combinations of syntenic genes, it has been assumed that inversion polymorphisms can be adaptive. For paracentric inversions, many studies with dipterans have been undertaken to link paracentric inversion polymorphism to aspects of the environment in which the particular insect exists.

Pericentric Inversions

Pericentric inversions result from breaks in each arm of a chromosome that rejoin after the excised piece containing the centromere has inverted. Pericentric inversion polymorphism was perhaps most famously studied in the morabine grasshopper, Keyacris scurra. White and coworkers used this rearrangement to develop adaptive topographies (defined by Sewell Wright) for various populations of K. scurra, that were on a saddle between adaptive peaks. The duplications and deletions that are the consequences of recombination within mutually pericentrically inverted segments seem to be largely avoided in insects bearing them at polymorphic frequencies. This is because the chromosomes are able to pair during meiosis without the inverted regions undergoing synapsis: so-called "torsion pairing."

Translocations

Translocations result from breaks in two chromosomes that allow exchange of pieces between the chromosomes. For breaks that are interstitial on the chromosome arms, a reciprocal translocation results, and in heterozygotes the translocated chromosomes synapse together at meiosis to form a quadrivalent (or a multivalent, if exchanges are more frequent). Multiple translocation heterozygosity has been observed, with resulting ring multivalents, in the cockroaches (Blattodea).

Centric Fusions

Centric fusions, or Robertsonian translocations, are special cases of translocation in which two breaks are very close to the centromeres of acrocentric chromosomes, causing the formation of a large metacentric or submetacentric chromosome and a very small remnant, which is lost. Centric fusions commonly distinguish chromosomal races or species in insects, but they are not seen to be maintained polymorphic in populations as frequently as they are in mammals. Centric fusions between sex chromosomes and autosomes results in the formation of neo-XY and Xj X2 Y systems in insects.

Dissociation

Dissociation, the reverse of fusion, involves the formation of two acrocentrics from a metacentric chromosome. Dissociation is rare because a donor centromere, a short arm, and a telomere are required; however this rearrangement was shown to occur in the dissociation that formed the two chromosomal races of the morabine grasshopper K. scurra.

Whole-Arm Interchanges

Whole-arm interchanges occur when chromosomal breaks and rejoinings near the centromeres of metacentric chromosomes result in the exchange of whole chromosome arms (Fig. 3). It has been noted that such exchanges distinguish races and species more frequently than reciprocal translocations, perhaps because the former maintain a sequence of coadapted genes in the arms concerned.

Complex Rearrangement

Complex rearrangements such as insertions, involving three or more breaks, have been noted in insect chromosomes, particularly after damage induced by radiation. Such work, particularly by H. Müller in D. melanogaster, led to each arm of the chromosome being defined as oriented from the centromere to the telomere.

See Also the Following Articles

Genetic Engineering • Parthenogenesis • Sex Determination Further Reading

Cook, L. G. (2000). Extraordinary and extensive karyotypic variation: A 48fold range in chromosome number in the gall-inducing scale insect Apiomorpha (Hemiptera: Coccoidea: Eriococcidae). Genome 43, 169—190. Crosland, M. W. J., and Crozier, R. H. (1986). Myrmecia pilosula, an ant with one pair of chromosomes. Science 231, 1278. Giles, E. T., and Webb, G. C. (1972). The systematics and karyotype of Labidura truncata Kirby, 1903 (Dermaptera: Labiduridae). J. Aust. Entomol. Soc. 11, 253—256. Gregg, P. C., Webb, G. C., and Adena, M. A. (1984). The dynamics of B chromosomes in populations of the Australian plague-locust, Chortoicetes terminifera (Walker). Can. J. Genet. Cytol. 26, 194—208. Hoy, M. (1994). "Insect Molecular Genetics." Academic Press, San Diego. John, B. (ed.). (1974-1990). "Animal Cytogenetics," Vol. 3, "Insecta." Bornträger, Berlin.

Jones, R. N., and Rees, H. (1982). "B Chromosomes." Academic Press, London.

King, M. (1993). "Species Evolution: The Role of Chromosome Change."

Cambridge University Press, Cambridge, U.K. Kipling, D. (1995). "The Telomere." Oxford University Press, Oxford, U.K. Shaw, D. D., Webb, G. C., and Wilkinson, P. (1976). Population cytogenetics of the genus Caledia (Orthoptera: Acridinae). II: Variation in the pattern of C-banding. Chromosoma 56, 169-190. White, M. J. D. (1973). "Animal Cytology and Evolution," 3rd ed.

Cambridge University Press, London. White, M. J. D. (1978). "Modes of Speciation." Freeman, San Francisco.

Frederick W. Stehr

Michigan State University

A chrysalis (plural chrysalids) is the pupa of a butterfly, usually belonging to the family Papilionidae, Pieridae, or

FIGURE 1 Pupa of the butterfly Papilio cresphontes, showing the strand of silk that holds some butterfly pupae in an upright position. (Photograph from the Teaching Collection, Department of Entomology, Michigan State University.)

Nymphalidae. It is commonly found suspended or hanging from a leaf, twig, or branch, or even a windowsill, arbor, or other suitable structure. Not all species in these families form chrysalids. For example, the parnassians in the Papilionidae and the wood nymphs (Satyrinae) in the Nymphalidae pupate in a minimal cocoon in grass, leaves, or litter. The pupae of the families Lycaenidae and Riodinidae are also not suspended and usually are concealed in leaves or litter.

In forming the chrysalis, the prepupal caterpillar has to perform the seemingly impossible maneuver of spinning the silk pad to attach its cremaster (caudal pupal hooks) while maintaining its grip; it then must molt the larval skin as it attaches to the silk pad. Members of the Pieridae and Papilionidae (except Parnassinae) secure the chrysalis in an upright position with a band of silk around the middle

Chrysalids are usually angular, with projections, tubercles, spines, and sometimes gold or silver flecks. They are often cryptically colored so that they blend into the surrounding materials but some, like the monarch chrysalis, are smooth with gold flecks. In emerging from the chrysalis the adult splits the chrysalis ventrally and dorsally at the anterior end, crawls out, and suspends itself from the pupal skin while its wings expand.

See Also the Following Article

Pupa and Puparium

Further Reading

Stehr, F. W. (ed.) (1987). "Immature Insects," Vol. 1. Kendall/Hunt, Dubuque, IA.

Stehr, F. W. (ed.) (1992). "Immature Insects," Vol. 2. Kendall/Hunt, Dubuque, IA.

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