Introduction
Sex determination is the biological process that directs an organism to develop as male, female, or occasionally an intermediate/intersex form. Mechanisms range from genetic systems (chromosomes and genes) to environmental cues (temperature, social factors). These mechanisms regulate development of primary sex organs and secondary sexual traits and ensure population-level sex ratios that support reproduction.
Sex Characters
Primary Sex Characters
Primary sex characters are the reproductive organs directly involved in gamete production and fertilization. In animals this includes testes, ovaries, sperm, ova, ducts and accessory glands. In plants, primary characters refer to male and female reproductive structures such as stamens (anthers) and pistils (ovary, style, stigma).
Secondary Sex Characters
Secondary sex characters are features not directly part of the reproductive system but that show sexual dimorphism and often influence mate choice — examples include body size differences, plumage or coloration, vocal differences, beards in humans, and floral differences in some plants. These traits are typically hormonally regulated and develop at sexual maturity.
History
Ideas about sex and its causes evolved from ancient natural philosophers to modern genetics. Aristotle proposed environmental influences (heat) on sex. With chromosome discovery in the 19th century and work by McClung, Stevens and Wilson in the early 1900s, the chromosomal basis (XX/XY) became established. Later molecular work identified specific sex-determining genes (for example, SRY in mammals) and clarified diverse systems across taxa.
Sex Determination — Major Systems
1. Environmental Sex Determination (ESD)
In ESD, external cues determine sex. The most studied form is temperature-dependent sex determination (TSD) in many reptiles: incubation temperature during critical windows produces males or females. Social environment also affects sex in some fishes (e.g., sequential hermaphrodites such as clownfish). Other environmental influences include photoperiod, pH, and nutrition.
2. Chromosomal Sex Determination
Chromosomal systems are widespread and include:
- XX–XY system (mammals, many insects): males XY (heterogametic), females XX (homogametic).
- ZZ–ZW system (birds, butterflies): females ZW (heterogametic), males ZZ (homogametic).
- XX–XO system (some insects like grasshoppers): males XO (single X), females XX.
- Haplodiploidy (honeybees): haploid males from unfertilized eggs, diploid females from fertilized eggs.
Molecular Basis of Sex Determination
1. Sex Determination by Genic Balance
Described in Drosophila, the genic balance model (Bridges) relies on the ratio of X chromosomes to sets of autosomes (X:A). When the ratio is ~1.0 (e.g., XX:2A) the pathway leads to female development; when ~0.5 (XY:2A) it leads to male development. Intermediate ratios produce intersex or abnormal phenotypes. The mechanism acts through regulation of key sex-switch genes and downstream splicing of transcription factors (e.g., transformer, double-sex).
2. Sex Determination by Y-linked Genes
In mammals, the presence of the SRY gene on the Y chromosome is pivotal. SRY encodes a transcription factor (testis-determining factor, TDF) that activates a cascade (e.g., SOX9) leading to testis formation and male differentiation. Loss or translocation of SRY can result in sex-reversal (XY females or XX males with SRY translocation).
Dosage Compensation
Dosage compensation balances expression of X-linked genes between sexes.
- Mammals: Random X-chromosome inactivation in females (Lyonization) — one X forms a Barr body; this equalizes gene dosage with XY males.
- Drosophila: Males hyper-activate their single X chromosome ~2-fold to match female XX expression.
- C. elegans: XX hermaphrodites partially down-regulate both X chromosomes to balance with XO males.
Sex Determination in Plants
1. Environmental
Many plants show plasticity: environmental cues such as day length, temperature, nutrient availability or plant density can shift sex expression. Monoecious and some dioecious species (e.g., cucumber, spinach) may alter flower sex ratios in response to conditions.
2. Chromosomal
Some dioecious plants use chromosomal sex determination. Examples:
- Silene latifolia (white campion): XY male, XX female.
- Papaya: has male, female and hermaphrodite forms controlled by sex chromosomes.
- Cannabis sativa: shows XY-type sex chromosomes in many varieties.
Evolution of X and Y Chromosomes
X and Y chromosomes likely evolved from a pair of ordinary autosomes. Acquisition of a sex-determining gene (e.g., proto-SRY) on one chromosome favored suppression of recombination around that locus to preserve favourable linkage. Over time, lack of recombination led to degeneration of the Y chromosome (gene loss, accumulation of repetitive DNA), while the X retained many essential genes. Independent sex chromosome evolution has occurred in different lineages; comparative genomics shows varying degrees of Y degeneration and gene content among species.
References & further reading: Core genetics and developmental biology textbooks; review articles on temperature-dependent sex determination, SRY and SOX9 pathways, Bridges' genic balance model, and plant sex chromosome evolution.
