Kenneth R. Lewis

The Chromosomal Basis of Sex Determination

Publisher Summary The study of sexuality cannot be divorced from a consideration of somatic differentiation within individuals and of variation between them. The basic problems are common to all these situations. The principal difference between sexual and somatic situations is that the former are directly involved in determining the organization of breeding. In consequence they are concerned with the structure and perpetuation of the breeding group rather than the survival of its individual members. The differences, therefore, are in relation to function and selection rather than determination. The chapter focuses on the chromosomal basis of sex determination, and discusses the genetic control of sex determination and the epigenetic control of sex determination. It is clear that while certain gene combinations can be recombined quite frequently, with impunity if not advantage, the breakup of others can cause little but grief except under the most unusual circumstances. The determinants controlling sexual dimorphism are of this kind. It is conceivable that large differences between the sexes, even in complex organisms, could be governed by a simple genetic difference at the hereditary level while, at the same time, many genes might be involved in determining the differences at the developmental level. Differentiation is essentially the development of phenotypic differences between cells with genetically identical nuclei. So, the differences between cells in respect of the action of many determinants are compatible with their genetic identity. The chapter also discusses the Sex Determination by Meiotic Segregation, by Somatic Segregation, by Somatic and Meiotic Segregation, and by Haplo-Diploidy.

Meiotic Behaviour of Special Chromosomes

The commonest system of genotypically controlled sex determination depends on the segregation of a difference at meiosis. This system may operate at the haploid level, as in many algae and bryophytes, or at the diploid level. From the standpoint of the meiotic system, the sporophytic phase in types with genotypically controlled haploid dioecism is comparable with the heterogametic sex of types showing diploid dioecism (Lewis 1961).

Structural Abnormalities at Meiosis

The variations on the meiotic theme described in the prevoius section were within the range of normality. The normal course of meiosis may also be considerably impaired as a consequence of abnormal genotypic or environmental conditions. And in this section we will consider those abnormalities which, though they do not depend on them, can lead to structural and numerical change.

Chromosome Duplication and Pairing

The early cytologists came to the conclusion that in a majority of organisms the leptotene chromosomes were single and that this singleness persisted at least until pachytene. This, in turn, led to the idea that chromosome duplication occurred during pachytene and not, as in mitotic tissues, during interphase. True there were claims to the contrary. Some workers held the leptotene chromosomes to be visibly double (see Rhoades 1961) but these claims were either treated as suspect or else could be fairly simply explained. Thus, in Tradescantia and Trillium it is possible to resolve half chromatid units with an ordinary light microscope (Nebel and Rutile 1936) so that the chromosomes at leptotene do show a bipartite structure. But, since the chromosomes at first metaphase are quadripartite, the leptotene threads can still be regarded as single in the sense that the prophase chromosomes of mitosis are double.

Meiosis in Species Hybrids

Species are differentiated to different degrees and in different ways. These include differences in gene frequency or type, chromosome morphology, chromosome number or various combinations of these. Consequently when species are crossed, either in nature or by experiment, the course of meiosis will be variably affected. Most species which produce hybrids with regular meiotic pairing and normal fertility appear to be differentiated primarily by genic factors. On the other hand most hybrids derived from parents differing in chromosome structure or number are at least partially sterile (Table 55).

Chromosome Movement and Segregation

So far we have discussed only those mechanical changes which affect the morphology of the chromosomes. Here we want briefly to indicate the bodily movements they undergo and which affect their distribution.

Meiosis in Organisms with Non-localised Centric Systems

So far we have dealt with chromosomes whose centric activity is strictly localised. Most of these are monocentric, a few dicentric, so that at the most only two centric units are functional at any one time and these occupy constant positions. There is, however, a number of species which posses a non-localised centric system.

Meiosis in Male Chorthippus

In the species of the genus Chorthippus, as in a majority of the genera in the sub-family Truxalinae, the mitotic chromosome complement of the male includes seventeen members. Sixteen of these are autosomes and can be matched into eight homologous pairs; the seventeenth chromosome is unpaired and represents a sex or X-chromosome. All these elements are individually distinguishable (Fig. 1). Six of the autosomes are metacentric and these are the longest chromosomes in the complement (pairs L1–L3); the remainder are acrocentric and include one pair of small chromosomes (S 8) and a range of medium pairs (M4–M7). The unpaired X-chromosome is the longest of the acrocentrics.

The Regulation of Meiosis

In the preceding sections we have seen something of the course of meiosis in a variety of organisms and something of the events leading to meiosis in some of them. We have seen that a standard system—that generalised sequence which the elementary student learns along with the generalised cell—obtains with but minor variations in by far the majority of them. We have also seen that numerical and structural changes in the chromosome can alter the pairing arrangements of chromosomes and, thereby, they may change the mechanical units of orientation and disjunction. Finally we have seen too that the most bizarre sequences characterise some organisms, especially certain male animals. And, although the matter is not always amenable to direct experimental analysis, we must conclude that differences in the nucleus are responsible for determining the variation in their own meiotic behaviour (Table 96).

Patterns of Variation in Chiasma Formation

In Chorthippus chiasmata form at various points along the length of the chromosome. But even in cases like this, with so-called random chiasma formation, it has long been evident that the pattern is not completely random (see pg. 207). However, in some organisms chiasmata occur exclusively, or nearly so, in very localised regions of the chromosome. Two such types of localisation are known—proximal and distal. In the first of these, chiasmata are restricted to the neighbourhood of the centromere, while in the second they are confined to short regions near the non-centric ends of the chromosome (Table 2, Figs. 27–30). This localisation rarely appears to be an absolute nuclear condition affecting all chromosomes equally. Thus in Stethophyma localisation is more complete in some bivalents than others and in some individuals than others (White 1936). Again in Bryodema, nine of the eleven acrocentric autosomal bivalents possess only one chiasma in the long arm; this forms adjacent to, or in the immediate neighbourhood of the centromere. But two chromosomes do not conform to this behaviour (White 1954). The smallest bivalent invariably forms a single distal chiasma, as too does the fifth bivalent, but in this case 64 of the 166 first meta-phases examined also had a proximal chiasma leading to the production of a ring bivalent. The same principle applies to plants too. Thus in Allium fistulosum there is an average of 1.7 non-localised chiasmata per cell.