Genetic factors and environmental conditions are the primary proximate determinants of individual gender and population-level sex ratios. Genetic sex determination is generally achieved by sex chromosomes, where males and females differ in the composition of one chromosome pair. The most familiar is the XY sex-determination system present in most mammals and insects, and in a few dioecious plants (e.g., Silene latifolia) in which males and females are separate individuals. In the XY system, males have two different kinds of sex-determining chromosomes (X and Y), and females have two of the same (both of type X). The WZ sex-determination system is found in birds and some insects. Here, the situation is reversed: females have two different kinds of sex chromosomes (W and Z), and males have two of the same kind of chromosomes (ZZ). Haplodiploidy, found in insects of the order Hymenoptera (ants, wasps, and bees) and some mites, is characterized by haploid individuals (which are males) and diploid individuals, which are usually females. Another kind of genetic determination of sex in plants results from an interaction between nuclear genes and mitochondrical genomes. Cytoplasmic male sterility (CMS) genes inhibit pollen production and are inherited through maternal lines. Nuclear male fertility restorer genes of biparental origin counteract CMS effects to restore pollen production.
Environmental sex determination occurs when the gender of an individual or the sex ratio of its offspring is determined by the environment during development. Such environmental effects on gender and the resulting sex ratio of a population are often interpreted as the outcome of natural selection. In this case, selection favors organisms (and their genes) that respond to environmental conditions by producing the gender or the sex ratio that has higher fitness than would the opposite sex or different sex ratios. There are numerous examples of such environmental sex determination that agree with predictions derived from sex ratio theory (described below). Some environmental factors that determine sex relate to resource availability, such as nutrients and water availability, but others result from interactions with other species (e.g., parasites and predators). Differential parental care and sex-dependent mortality rates can also affect sex determination and resulting sex ratios.
Table 1 Examples of sex ratio in case studies Taxonomic
Species group Sex ratio (proportion of males) Comments, mechanism, etc. References
Acer negundo Angiosperm
Salix repens Urtica dioica Ochradenus baccatus Phillyrea angustifolia Silene vulgaris
Tigriopus californicus Oryzias latipes
Ficedula albicollis Acrocephalus sechellensis Homo sapiens
Angiosperm Angiosperm Angiosperm
0.394 (streamside, favorable habitat)
0.619 (off-stream, poor habitat)
0.25-1.00 (among populations)
Angiosperm 0.521 (competition present)
0.496 (competition absent) Copepod 0.515
Fish 0.27 (fish exposed to pesticides) to
0.57 (low level of pesticides) Bird 0.54 to 0.38
Bird 0.13-0.77 among unhelped breeding pairs
Mammal 0.515 (1950) to 0.512 (1993) Mammal 0.35
Gynodioecious; frequency-dependent [5, 6] selection on sex ratio, depending on metapopulation structure, rarity of females, and scarcity of pollinators
Monoecious; ratio of male flower heads 
Parental inheritance of sex 
determination Environmentally induced sex change 
Offspring sex ratio, depending on 
reproductive effort of male Offspring sex ratios are adjusted in 
response to quality of territory Decline in male to female ratio at birth in 
industrial countries. Soveso, Italy, 7 years after chemical  accident
 Dawson TE and Ehleringer JR (1993) Gender-specific physiology, carbon Isotope discrimination, and habitat distribution In boxelder, Acer negundo. Ecology 74: 798-815;  De Jong TJ and Klinkhamer PGL (2002) Sex ratio in dioecious plants. In: Hardy ICW(ed.) Sex Ratios - Concepts and Research Methods, pp. 349-364. Cambridge: Cambridge University Press;  Wolfe LM and Shmida A (1997) The ecology of sex expression in a gynodioecious Israeli desert shrub (Ochradenus baccatus) Ecology 78(1): 101-110;  Pannell JRand OjedaF (2000) Patterns of flowering and sex-ratio variation in the Mediterranean shrub Phillyrea angustifolia (Oleaceae): Implications for the maintenance of males with hermaphrodites. Ecology Letters 3: 495-502;  Olson MS, McCauley DE and Taylor DR (2005) Genetics and adaptation in structured populations: Sex ratio evolution in Silene vulgaris. Genetica 123: 49-62;  McCauley DE et al. (2000) Population structure influences sex ratio evolution in a gynodioecious plant. American Naturalist 155: 814-819;  Holzapfel C and Mahall BE (1999) Bidirectional facilitation and interference between shrubs and annuals in the Mojave Desert. Ecology 80:1747-1761 ;  Voordouw MJ, Robinson HE, and Anholt BR (2005) Paternal inheritance of the primary sex ratio in a copepod. Journal of Evolutionary Biology 18(5): 1304-1314;  Teather K, Jardine C, and Gormley K (2005) Behavioral and sex ratio modification of Japanese medaka (Oryzias latipes) in response to environmentally relevant mixtures of three pesticides. Environmental Toxicology 20:110-117;  Ellegren H, Gustafsson L, and Sheldon BC (1996) Sex ratio adjustment in relation to paternal attractiveness in a wild bird population. PNAS 93:11723-11728;  Komdeur J, etal. (1997) Extreme adaptive modification in sex ratio of the Seychelles warbler's eggs. Nature 385: 522-525;  Davis DL, Gottlieb MB, and Stampnitzky JR (1998) Reduced ratio of male to female births in several industrial countries: A sentinel health indicator? Journal of the American Medical Association 279:1018-1023; and  Mocarelli P, et al. (1996) Change in sex ratio with exposure to dioxin. Lancet 348: 409-410.
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