Origin of a Giant Sex Chromosome

Chromosome size and morphology vary within and among species, but little is known about the proximate or ultimate causes of these differences. Cichlid fish species in the tribe Oreochromini share an unusual giant chromosome that is ∼3 times longer than the other chromosomes. This giant chromosome functions as a sex chromosome in some of these species. We test two hypotheses of how this giant sex chromosome may have evolved. The first hypothesis proposes that it evolved by accumulating repetitive elements as recombination was reduced around a dominant sex determination locus, as suggested by canonical models of sex chromosome evolution. An alternative hypothesis is that the giant sex chromosome originated via the fusion of an autosome with a highly repetitive B chromosome, one of which carried a sex determination locus. We test these hypotheses using comparative analysis of chromosome-scale cichlid and teleost genomes. We find that the giant sex chromosome consists of three distinct regions based on patterns of recombination, gene and transposable element content, and synteny to the ancestral autosome. The WZ sex determination locus encompasses the last ∼105 Mb of the 134-Mb giant chromosome. The last 47 Mb of the giant chromosome shares no obvious homology to any ancestral chromosome. Comparisons across 69 teleost genomes reveal that the giant sex chromosome contains unparalleled amounts of endogenous retroviral elements, immunoglobulin genes, and long noncoding RNAs. The results favor the B chromosome fusion hypothesis for the origin of the giant chromosome.

The Abdominal Epidermis of Calliphora Erythrocephala (Diptera)

The extremely rapid growth of feeding larvae of Calliphora is achieved with a hypertrophy of larval epidermal cells and concomitant polytene specialization of their nuclei. Within the epidermis of each abdominal segment are differentiated sense organs of the campaniform and a more complex - possibly chemosensory - kind in a constant pattern; there is no post-embryonic differentiation of new sense organs before metamorphosis, while the cuticle-secreting cell of these sense organs also becomes polytene. The polytene organization of epidermal nuclei differs from the classical giant chromosome structures of lower dipteran larval cells. Calliphora epidermal polytene chromosomes irregularly fragment into oligotene or smaller fibrils which, through apparently non-specific reassociations, form a nuclear reticulum of which the banded polytene regions are a part. This loose reticular organization is unlikely to reflect a condition of genetic activity. Photometric densitometry shows the period of polytene DNA replication in general epidermal nuclei is closely correlated with the period of larval growth. At hatching, epidermal nuclei are diploid, with the 2C DNA content. Through a series of endoreduplications, such that at any stage nuclei tend to fall into a series of DNA classes with a percentage falling in between, the largest polytene nuclei reach about 256C by the end of the feeding stage. During the feeding stage, whole mount autoradiography shows a very slow continuation of DNA synthesis in some epidermal nuclei; that is, after cessation of larval growth. DNA synthesis in cupologen nuclei of the campaniform organs ceases after secretion of the third instar cuticle at the end of the second instar. The polytene cycle responsible for nuclear DNA increase is discussed in the context of these data, and a functional correlation drawn for the rate of polytene increase in the epidermal nuclei and the cupologen nuclei. This correlation suggests active control of the cycle, and raises the question of the significance of the polytene cell cycle.

THE SALIVARY GLAND CHROMOSOMES OF SIX CLOSELY RELATED BLACK FLIES NEAR EUSIMULIUM CONGAREENARUM (DIPTERA: SIMULIIDAE)

The salivary gland chromosomes of nearctic black flies which form a natural group in Eusimulium close to E. congareenarum were analyzed in detail. Comparisons of their giant chromosome banding patterns disclosed six cytological segregates in two subgroups; subgroup A, with E. innocens, E. anatinum, E. congareenarum, and a cytologically distinct form near the latter designated E. congareenarum 'b'; subgroup B, with E. excisum and E. rivuli. Within each subgroup closely related species differ at least by (1) two or three interspecific inversions, (2) the intraspecific specific inversions present, and (3) the details of the X and Y chromosomes. The differences between the subgroups include (1) the position of the nucleolus, (2) the identity of the sex chromosomes as either the first or third pair, and (3) about 15 interspecific inversions between E. congareenarum and E. excisum, the most closely related species from either subgroup. The phylogenetic interrelationships have been traced by means of the interspecific inversions.