In Y-centered sex determination, the SRY gene is the main gene in determining male characteristics, but multiple genes are required to develop testes. Haplodiploidy Haplodiploidy is found in insects belonging to Hymenoptera , such as ants and bees. In XY mice, lack of the gene DAX1 on the X chromosome results in sterility, but in humans it causes adrenal hypoplasia congenita. For example, while having an XY format, Xiphophorus nezahualcoyotl and X. Transcaucasian mole voles Ellobius lutescens also have a form of XO determination, in which both sexes lack a second sex chromosome.
Generally in this method, the sex is determined by amount of genes expressed across the two chromosomes. For example, while having an XY format, Xiphophorus nezahualcoyotl and X. Diploid individuals are generally female but may be sterile males. The ZW sex-determination system is reversed compared to the XY system: The ZZ become males, but the WW are not viable and are not brought to term. Lizards order Squamata, suborder Sauria are particularly fascinating because the distribution of sex-determining mechanisms shows no clear phylogenetic segregation. This may be significant for the development of eusociality , as it increases the significance of kin selection , but it is debated. These autosomes are thought to have evolved sex-determining loci that eventually developed into the respective sex chromosomes once the recombination between the chromosomes X and Y or Z and W was suppressed. This is due to the fact that the haploid eggs double their chromosomes, resulting in ZZ or WW. In this paper, we review the current state of knowledge of sex chromosomes in lizards and the distribution of sex determining mechanisms and sex chromosome forms within and among families. In TSD, the temperature at which an embryo develops determines whether it becomes male or female, whereas in GSD, the sex of the animal is predetermined by its sex chromosomes and usually is not altered by thermal conditions. When meiosis occurs in the sporophyte generation of the life cycle, the sex chromosomes known as U and V assort in spores that carry either the U chromosome and give rise to female gametophytes, or the V chromosome and give rise to male gametophytes. In Y-centered sex determination, the SRY gene is the main gene in determining male characteristics, but multiple genes are required to develop testes. In this system, most females have two of the same kind of sex chromosome XX , while most males have two distinct sex chromosomes XY. We establish for the first time an association between the occurrence of female heterogamety and TSD within lizard families, and propose mechanisms by which female heterogamety and TSD may have co-evolved. We suggest that lizard sex determination may be much more the result of an interplay between sex chromosomes and temperature than previously thought, such that the sex determination mode is influenced by the nature of heterogamety as well as temperature sensitivity and the stage of sex chromosome degeneration. Males cannot have sons or fathers. ZW sex-determination system The ZW sex-determination system is found in birds, some reptiles, and some insects and other organisms. Approximately 1, species of lizards have been karyotyped and among those, fewer than species have sex chromosomes, yet they display remarkable diversity in morphology and degree of degeneration. The X and Y sex chromosomes are different in shape and size from each other, unlike the rest of the chromosomes autosomes , and are sometimes called allosomes. The eggs of sea kraits have seldom been found so little is known about the conditions under which they develop. In Bombyx mori the commercial silkworm , the W chromosome carries the female-determining genes. Although it is an XY system, the platypus' sex chromosomes share no homologues with eutherian sex chromosomes. However, this is unlikely as all snakes, terrestrial and aquatic, that have been tested have GSD Modi and Crews Unfertilized eggs develop into haploid individuals, which are the males. The high diversity of sex chromosomes as well as the presence of species with TSD, imply multiple and independent origins of sex chromosomes, and suggest that the mechanisms of sex determination are extremely labile in lizards. The 0 denotes the absence of a second sex chromosome.
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