Linkage group-chromosome correlation in a mosquito: translocations in Culex tritaeniorhynchus

An alternative method for isolating homozygotes of autosomal translocations in the mosquito Culex tarsalis

An alternative pseudolinkage procedure for isolating homozygotes of autosomal translocations has been developed with the mosquito Culex tarsalis (Coquillet). The first step was to induce a translocation heterozygote in a population that was marked with recessive mutants. Interbred translocation heterozygotes produced translocation homozygotes that were phenotypically different from their translocation heterozygote and normal siblings. Thus, a translocation homozygote line of this species was selected and established in shorter time and with less effort than by prior pseudolinkage procedure.

Translocations, inversions and correlation of linkage groups to chromosomes in the mosquito Culex pipiens fatigans

Eight sex-linked translocations and two pericentric inversions were artificially induced and isolated in the mosquito Culex pipiens fatigans. Following extensive studies on the genetics and cytology of these aberrations, correlations between the three pairs of chromosomes and the three linkage groups were established. Chromosomes 1 (shortest), 2 (longest) and 3 (intermediate) are correlated to linkage groups I, II and III respectively. These conclusions are in conflict with an earlier publication. A secondary constriction was observed on the m (but not on the M) chromosome near the centromere. Its significance is not yet clear. The genetic and cytological data indicate that the gene y may be located on the long arm and the gene ru on the short arm of chromosome 2.

[Sterility due to aberrations]

A decrease of fertility can be caused by chromosomal aberrations of two different types: 1) Inversions and transpositions Aneuploid gametes will only be formed, if crossingover had happened between the breakpoints. A possible regulation against this sterility is avoiding any crossingover in the organism. Especially mechanisms in oogenesis and spermatogenesis are developed against paracentric inversions only. 2) Reciprocal translocations Aneuploid gametes are the result of adjacent segregation of the chromosomes involved in the translocation. The percentage of sterility is correlated with the relation of alternate and adjacent segregation. At random segregation results a relation of 2∶4 between euploid and aneuploid gametes resp. 66% sterility. But in literature there are described all possible relations between euploid and aneuploid gametes; this not only in different genera but also in populations of a species (per example in crossing experiments). These facts, especially the results of crossing experiments beget the hypothesis, that segregation of the chromosomes involved in a translocation is under strict genetic control. We postulate, that it is attributable to a single, Mendelian factor, called "sg". This factor exists in a series of multiple alleles causing different relations between alternate and adjacent segregation. The allelomorph causing a higher fertility is dominant over the allelomorph, producing less euploid gametes. The percentage of sterility resp. fertility due to a translocation is not constant, but can be changed by mutual events. The individuum possessing the allele for more fertile gametes - without respect whether homo- or heterozygotous - will produce more offspring than another individuum with lower regulating alleles. So the alleles for sterility will be lost in the course of evolution. Finally, only individuals of a species with complete fertility (sg(4)/sg(4)) will survive in spite of segmental interchange (like Oenothera ssp., Periplaneta americana etc.). These mechanisms of regulation against sterility due to chromosomal aberrations are of interest in view of pest control with genetic methods.

Sex-linked translocations, semisterility and linkage alterations in the mosquito Aedes aegypti

Seven sex-linked translocations were artificially induced and isolated in the mosquito Aedes aegypti. All translocations have passed through four generations. Each translocation heterozygote shows characteristic sterility with small variation. Egg hatches of the seven translocation heterozygotes ranged from 18% – 71%. Two translocations are TMtype, i.e. linked to the M chromosome, and the remaining five are Tmtype, i.e. linked to the m chromosome. Possibilities of using both types of translocation for genetic manipulation of field population are discussed. The three linkage groups (I,II,III) have been correlated to the three pairs of chromosomes (1, 2, and 3). For the first time in mosquitoes, translocations have been utilized to correlate a specific gene to a particular segment of a chromosome. The blt locus has been assigned to the small terminal segment distal to the secondary constriction on chromosome 3.

The genetics of delta, a dominant sex-linked mutant of the mosquito, Culex tritaeniorhynchus

Delta, a new mutant of the mosquito Culex tritaeniorhynchus is described. Genetic analysis revealed that this gene is dominant and closely linked to sex. The possibility of using this gene to detect cryptic variabiltiy in the sex chromosomes of natural populations of this mosquito is discussed.

Sterility enhancement in the mosquito. Culex tritaeniorhynchus

The sterilities of stocks with chromosomal aberrations were enhanced by re-irradiation. Males heterozygous for two independent aberrations were synthesized and their sterilities determined. The possible advantages of the double heterozygote males over either completely sterile males or those heterozygous for only one aberrational complex are discussed.