5.1 Animal and plant breeding materials

Breeding materials are the living materials, records and equipment used to produce the next generation of improved animals or plants. In animals they include breeding stock, semen, eggs (ova) and embryos; in plants they include seeds, pollen, cuttings, tubers and tissue culture plantlets. The quality of these materials decides how much genetic improvement a breeding programme achieves, so they must be selected from proven parents, handled correctly and stored under the right conditions. Example: an artificial insemination service in Kuresoi keeps frozen bull semen straws in liquid nitrogen tanks and delivers them to dairy farmers on motorbikes within hours of a call.

Animal breeding materials

• Breeding stock: Selected males (sires, bulls, bucks, boars, cocks) and females (dams, cows, does, sows, hens) with known performance and pedigree.
• Semen: Collected from tested sires, diluted with an extender, packed in straws and frozen at −196 °C in liquid nitrogen for artificial insemination. One collection can serve hundreds of females.
• Sexed semen: Semen sorted to carry mostly X-bearing sperm, giving about 90% heifer calves for dairy farmers.
• Ova and embryos: Eggs and embryos collected from superior females after superovulation, used in embryo transfer and in vitro fertilisation.
• Fertile eggs: Hatching eggs from selected parent flocks, used in poultry breeding.
• Records: Pedigree, milk yield, growth rate, fertility and health records used to choose parents.

Plant breeding materials

• Seeds: Certified seed of known variety and germination rate, the main material for most field crops.
• Pollen: Collected from selected male parents for controlled hand pollination; stored dry and cool for short periods.
• Vegetative materials: Cuttings (cassava, sweet potato vines, tea), suckers (banana), tubers (potato), bulbs (onion) and budwood or scions for grafting (avocado, mango).
• Tissue culture plantlets: Disease-free plants grown from small pieces of tissue on sterile nutrient media.
• Germplasm collections: Gene banks storing seeds and tissues of many varieties and wild relatives as a source of genes.

Breeding equipment

5.1.1 Sex determination

Sex determination is the mechanism that decides whether an individual develops as male or female. In most animals it is decided by sex chromosomes at fertilisation, while in some it depends on the environment. Breeders need to understand sex determination to predict the ratio of males to females and to use tools such as sexed semen.

XX-XY system

Found in mammals, including humans and livestock, and in fruit flies. Females are XX (homogametic, producing only X eggs); males are XY (heterogametic, producing X and Y sperm in equal numbers). The father's sperm therefore decides the sex of the offspring.

Parents: XX (female) × XY (male)

Gametes: X × X and Y

Offspring: 1 female : 1 male. In mammals, the SRY gene on the Y chromosome switches on testis development.

ZW system

Found in birds, some reptiles and butterflies. Males are ZZ (homogametic) and females ZW (heterogametic), so the hen decides the sex of the chick. Breeders use sex-linked feather colour or feathering rate genes on the Z chromosome to sex day-old chicks.

XO system

Found in grasshoppers and some other insects. Females are XX and males have a single X (XO), so males have one chromosome fewer than females.

Haplodiploidy

Found in bees, wasps and ants. Fertilised eggs (diploid) develop into females (queens or workers) and unfertilised eggs (haploid) develop into males (drones). Beekeepers recognise a queen that has run out of sperm when the colony produces mainly drones.

Environmental sex determination

In many reptiles, such as crocodiles and most turtles, the incubation temperature of the egg decides sex. In some turtles, warm nests produce mainly females and cool nests mainly males, so rising temperatures threaten sex ratios.

Sex determination in plants

• Hermaphrodite flowers: Most flowering plants carry male and female parts in the same flower (beans, peas and hibiscus).
• Monoecious plants: Separate male and female flowers on the same plant, as in maize (tassel and cob), cucumbers and pumpkins.
• Dioecious plants: Separate male and female plants, as in date palm, asparagus and many pawpaw varieties (pawpaw also has hermaphrodite plants). Pawpaw growers plant several seedlings per hole and remove extra males after flowering.
• Sex chromosomes in plants: Some dioecious plants such as Silene have an XX-XY system.