Unusual sex chromosome inheritance in mammals

Abstract

The male has proven to be the heterogametic sex in all mammals studied so far. As is well known, the males usually have the sex chromosomes XY and the females XX. In recent years, however, many exceptions from this general pattern have been discovered. With our present knowledge, the different sex chromosome mechanisms in mammals may be divided into five main groups, and the first of them into subgroups, as follows: (i) Species with XX/XY sex chromosomes: (a) X of original size (see below), Y small; (b) X large, Y small; (c) X large, Y large: (i) end-to-end association of X and Y at male meiosis, (ii) chiasma between X and Y at male meiosis. (ii) Species with XX/XY 1 Y 2 sex chromosomes. (iii) Species with X 1 X 1 X 2 X 2 /X 1 X 2 Y sex chromosomes. (iv) Species with complicated or unknown mechanisms for sex determination. (v) Species with mosaicism of the sex chromosomes, but apparently with an XX/XY mechanism for sex determination. The present contribution will mainly deal with unusual sex chromosome inheritance, that is the groups (ii), (iii) and (iv) above, but the other two groups will also be briefly discussed and examples will be given. Recently Raicu, Kirillova & Hamar (1969) described a new sex chromosome mechanism ( X 1 X 1 X 2 X 2 /X 1 X 2 Y 1 Y 2 ) in the vole Microtus arvalis , but this observation was not confirmed by Schmid (1969), who found an ordinary XX/XY mechanism with both X and Y readily identifiable and of ‘normal’ size, the X comprising 5.6% of ( n A + X) and Y being the smallest chromosome of the complement. Late DNA replication was demonstrated in the allocyclic X and in the Y. Also Wolf (1969) found normal sex chromosomes in this species with no multivalents at male meiosis.

The chromosome complement of Sorex granarius–the ancestral karyotype of the common shrew (Sorex araneus)?

“…The sex chromosome complement is invariant and similar to other species within what is now known as the S. araneus complex (Hausser et al, 1985), these all have an XX/XY1Y2 system (Fredga, 1970). The remaining six autosome arms (a, b, c,f t, u) are always present combined in the metacentrics af bc and tu in the common shrew karyotype.…”

Fertile XO males and females in the varying lemming, Dicrostonyx torquatus pall. (1779)

“…They vary in number and at meiosis often occur as univalents. It is known that in some mammals the Y can be completely separated from the X at late prophase and metaphase 1 (Fredga, 1971). It may be possible, therefore, that the Y of varying lemming being similar to the B's in size and meiotic behaviour is concealed among them.…”

Temporal and spatial distribution of Rad51 protein in spermatocytes of the common shrew Sorex araneus L. (Soricidae, Eulipotyphla)

“…This is rather different from the behaviour of the shrew X chromosome in female somatic cells. Fredga (1970) and Pack et al (1993) demonstrated that in somatic cells of female shrews the arm e was late replicating and apparently inactivated as an unfused X chromosome in other species, but inactivation did not spread to the autosomal arm d.…”

“…The X in Sorex araneus represents a fusion between the true mammalian X and an autosome (Sharman, 1956(Sharman, , 1991Fredga, 1970;Pack et al, 1993). It has been shown that sex chromosomes in mammals differ from autosomes both in the timing and pattern of pairing and recombination (Ashley, 2002).…”