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THE GENETICS OF SEX IN LEPIDOPTERA

1938; Wiley; Volume: 13; Issue: 2 Linguagem: Inglês

10.1111/j.1469-185x.1938.tb00509.x

1469-185X

E. A. Cockayne,

Lepidoptera: Biology and Taxonomy

Summary 1. There are no hormones, which influence the development of the male or female secondary sexual characters, circulating in the blood, but it is probable that the corpus allatum secretes a hormone, identical in both sexes, causing the glandulae accessoriae in the male to complete their development and in the female causing the ova to mature. 2. The female sex is heterogametic, and has an X ‐ and a Y ‐chromosome. Sex is determined by the correct balance between factors for maleness in the X ‐chromosome and for femaleness in the autosomes and in the cytoplasm. Their valencies differ in different species and in different races of the same species. If the valency of the X ‐chromosome greatly exceeds that of the cytoplasm, complete sex reversal may occur and an insect constituted XY may be indistinguishable from a normal male. When the valencies are more nearly equal, male intersexes may be produced, and these may be due to relative weakness of the cytoplasm or of the autosomes. The converse may also occur and females by sex‐reversal or female intersexes may be produced. Examples of sex reversal and intersexuality occur in crosses between different races of Lymantria dispar and in many primary hybrids. Intersexes are also produced by crossing a male of the bisexual race of Solenobia triquetrella and a parthenogenetic female of the same species. The cells in different parts of such intersexes may be haploid, diploid, or polyploid. Intersexes occur in many secondary hybrids and possibly in L. dispar , and the condition is due to lack of balance between the X ‐chromosome and the autosomes. A varying number of the latter fail to conjugate and the insects approximate to the triploid state. There will be less than three but more than two sets of autosomes to three X ‐ or to two X ‐ and one Y ‐chromosome, giving a ratio of X ‐chromosomes to autosomes approaching 1:1·5, about midway between that of a male and a female. The ratio may vary in different parts of the same insect, and this may account for the coarse mosaic of male and female parts. Intersexes are found in several species of Lycaenidae. They are restricted to certain colonies, in which there is an excess of females, and their gonads and other sexual organs are female. The cause is unknown. Families consisting of females only or containing a large excess of females are found in various species, and three causes for this are known. In Abraxas grossulariata it is due to a gene, which causes the passage of the X ‐chromosome to the second polar body during the maturation of all or nearly all the ova. In Talaeporia it is due to non‐disjunction, and in Lymantria dispar to a dominant lethal gene, which kills all or nearly all the males. In Acraea and Hypolimnas the cause is unknown. 3. Several sex‐linked recessive characters and at least one sex‐linked dominant character are known in Lepidoptera, but in comparison with autosomal characters they are rare. Sex‐limited characters occur in the females of various species, and appear to be determined by a dominant autosomal gene acting in conjunction with a gene in the Y ‐chromosome or to a gene in the Y ‐chromosome acting alone. In Pygaera a sex‐limited inherited cancer occurs and kills every male larva. It is transmitted by all the females to all their sons. Federley thinks that it is due to a recessive gene in the Y ‐chromosome, inhibited by the presence of an X ‐chromosome, and the action of the gene is to stimulate mitosis. He believes that the tumour is derived from the triploid polocyte. This in the female is XXY , and though the gene is present in the Y it is inhibited by the two X ‐chromosomes. In the male the polocyte is XYY and the one X fails to inhibit the action of the gene in the two Y ‐chromosomes. The gene, being in the Y , is transmitted by every female. 4. Gynandromorphism. ( a ) Boveri's hypothesis. A delay in the passage of the spermatozoon after entering the egg, so that the egg has begun to divide before the sperm nucleus reaches it and only one‐half is fertilized. ( b ) Only one polar body is extruded; a second maturation division takes place, and of the two ootids so formed one is fertilized and gives rise to female parts, while the other, unfertilized, gives rise to male parts. Whiting & Whiting have shown that this occurs in Hymenoptera. ( c ) Morgan's first hypothesis. More than one spermatozoon enters the egg, and while one fertilizes it and forms female parts, the male parts are formed by the development of one or more of the others. ( d ) Morgan's second hypothesis. An X ‐chromosome is eliminated at the first division of the fertilized egg, or at some subsequent cell division. ( e ) By non‐disjunction an XXX individual is produced, and at the first division of the fertilized egg or at some subsequent division one of these becomes separated and passes to one cell and the other two pass to the other cell. In Lepidoptera male parts would arise from the cell with two X ‐chromosomes and female parts from the cell with only one. ( f ) From a binucleate ovum, each nucleus fertilized by a different spermatozoon. The two nuclei may be egg nuclei as in Abraxas or one may be an egg nucleus and the other a polar nucleus as in Bombyx mori. ( g ) Mechanical shock to the pupa as it is casting its larval skin. In Lepidoptera most gynandromorphs probably arise from binucleate eggs of the one or other kind. The elimination of an X ‐chromosome has been proved to produce gynandromorphism in Abraxas , but can only be proved when a sex‐linked character is involved and few of these are known. One example of a mosaic arising in the way suggested by Morgan in his first hypothesis is known, so that it is possible that some gynandromorphs are produced in this way. Somatic non‐disjunction is the most likely explanation of those Lepidopterous gynandromorphs, of which only a small portion is male. These would start as females ( XY ) and the male parts would be XXY. The production of gynandromorphs by shock requires confirmation. 5. Facultative parthenogenesis has occurred in many species of Lepidoptera belonging to different families. Both sexes are present in equal numbers in the offspring and their cells are diploid. It is probable that division of chromosomes takes place without cell cleavage and that males are XX and females YY in constitution. Obligatory parthenogenesis is common in Psychidae and Tineidae and occurs occasionally in Lymantridae. The parthenogenetic females are tetraploid, but the tetraploid condition is produced in two different ways at least.