WHAT does "sex" mean to a fungus? Sexual reproduction involves the fusion of haploid nuclei bearing only one set of chromosomes to form a diploid nucleus or zygote with the full complement of paired chromosomes. This can go on to produce new haploid nuclei by a series of cell divisions known as meiosis. These basic facts about sex remain the same in all fungi, although there are a range of different life cycles; in some species the predominant life form is diploid, whereas most fungi are haploid during the vegetative phase of growth. Sexual reproduction provides a new mix of genes giving fungi an evolutionary advantage in changing environmental conditions.

Many fungi can also reproduce asexually, forming spores by mitosis, without the need for sex, but these spores usually contain exactly the same genetic information as the parent fungus and so do not generate variation. Asexual reproduction has been likened to a man buying 100 tickets in a raffle, only to find that they all have the same number. A sexual punter may be able to afford fewer tickets, but at least they will all have different numbers.

The fruiting bodies and sexual spores formed by many fungi are also highly resistant to adverse environmental conditions, and can remain dormant for long periods, waiting for better times. Sexual reproduction may also lead to the repair of random genetic damage in nuclei as genes recombine.

Conditions have to be right before fungi undergo sex. Temperature, humidity, light and nutrition must be favourable. Frequently fungi initiate sex when they are stressed, provided that they have stored up enough nutrients during vegetative growth.

Genetic conditions must also be conducive to sex. Some fungi are homothallic, being able to have sex by themselves, with identical haploid nuclei fusing to form a diploid zygote. Other heterothallic fungi require the presence of a compatible partner. For sex to be successful, individuals must be of different mating type. At its simplest, individuals may be + or - types. But they may further act as "males" and "females", according to the contribution they make towards the formation of reproductive structures, and hundreds of different mating types exist in some mushrooms and toadstools. In heterothallic fungi, the thread-like elements that constitute the vegetative body of a fungus, the hyphae, must contain compatible haploid nuclei.

These fuse together as a prelude to sex, so that compatible nuclei within the hyphae themselves fuse together. Often specialised structures are formed to bring about such hyphal fusion. Compatible haploid nuclei within fused hyphae may then coalesce immediately, or form a long-lived partnership before finally yielding to sex, as happened in the Basidiomycotina, the mushrooms and toadstools.

Given favourable conditions for sex, fungal hyphae aggregate and differentiate to form novel reproductive structures in the process of sexual morphogenesis. This involves the development of two main hyphal systems. One is concerned with the production of the tissues of the fruit body itself. The other, more limited, system develops within the cushioned matrix of the growing fruit body and is where true "sex" takes place, with the fusion of haploid nuclei followed by meiosis giving rise to sexual spores. The growth of both hyphal systems is highly coordinated, to ensure that a characteristic fruit body shape results, favourable for the dispersal of spores. Sexual spores are eventually ejected from the fruit bodies, to be picked up by wind currents and spread into new areas potentially favourable for growth.