Aphid - Reproduction

Reproduction

Some aphid species have unusual and complex reproductive adaptations, while others have fairly simple reproduction. Adaptations include having both sexual and asexual reproduction, creation of eggs or live nymphs and switches between woody and herbaceous types of host plant at different times of the year.

Only females are present in the population (although, a few species of aphids have been found to have both male and female sexes). The overwintering eggs that hatch in the spring result in females, called fundatrices. Reproduction is typically parthenogenetic and viviparous. Eggs are parthenogenetically produced without meioisis and the offspring are clonal to their mother. The embryos develop within the mothers' ovarioles, which then give live birth to first instar female nymphs (viviparous). The offspring resemble their parent in every way except size, and are called virginoparae.

This process iterates throughout the summer, producing multiple generations that typically live 20 to 40 days. Thus one female hatched in spring may produce thousands of descendants. For example, some species of cabbage aphids (like Brevicoryne brassicae) can produce up to 41 generations of females.

In autumn, aphids undergo sexual, oviparous reproduction. A change in photoperiod and temperature, or perhaps a lower food quantity or quality, causes females to parthenogenetically produce sexual females and males. The males are genetically identical to their mothers except that they have one less sex chromosome. These sexual aphids may lack wings or even mouthparts. Sexual females and males mate, and females lay eggs that develop outside the mother. The eggs endure the winter and emerge as winged or wingless females the following spring. This is, for example, the life cycle of the rose aphid (Macrosiphum rosae, or less commonly Aphis rosae), which may be considered typical of the family. However in warm environments, such as in the tropics or in a greenhouse, aphids may go on reproducing asexually for many years.

Some species produce winged females in the summer, sometimes in response to low food quality or quantity. The winged females migrate to start new colonies on a new plant, often of quite a different kind. For example, the apple aphid (Aphis pomi), after producing many generations of wingless females on its typical food-plant, gives rise to winged forms which fly away and settle on grass or corn-stalks.

Some aphids have telescoping generations. That is, the parthenogenetic, viviparous female has a daughter within her, who is already parthenogenetically producing her own daughter. Thus a female's diet can affect the body size and birth rate of more than two generations (daughters and granddaughters).

Aphid reproduction jargon:

Heteroecious – host alternating

• Egg
• Fundatrix (foundress from the first egg)
• Fundatrigeniae (daughter clones)
• Emigrant (winged female; in spring, winged aphids migrating from primary hosts infest Poaceae)
• Apterous exule (wingless female)
• Alate exule (winged female)
• Gynoparae (produce sexual females)
• Male
• Oviparae (sexual females that mate with the males)

Autoecious – single host

• Egg
• Fundatrix
• Apterous exule
• Alate exule
• Sexuparae (Parthenogenetic viviparous females of aphids giving rise to the sexual generation and usually developing on the secondary host, the alate forms migrating to the primary host at the end of the summer (holocyclic and heteroecious aphids).)
• Oviparae
• Males

Within these two host life cycles, there exist 2 other forms of life cycle, holocyclic (sex involved; will lead to egg production which facilitates overwintering), anholocyclic (no sex or egg involved, reproduce parthenogenetically), androcyclic (reproduction at end of season by parthenogenesis to produce males to contribute to holocyclic phase).

The bird cherry-oat aphid is an example of a host alternating species (as implied by the double barrelled name), that starts its life cycle with a large, highly fecund fundatrix. Her offspring then proceed to grow and produce emigrants which develop on the bird cherry before flying to the oat species where they continue feeding. The subsequent apterous exules feed solely on the oats and eventually lead to growth of gynoparae which will return to the bird cherry where they will produce males and oviparae, which in turn will reproduce, giving eggs for the next year.

In heteroecious species, the aphids spend winter on tree or bush aka primary host, in summer they migrate to their secondary host on a herbaceous plant, then the gynoparae return to the tree in autumn. The pea aphid has a primary host of a perennial vetch and secondary of the annual pea. This is likely due to the decline of food quality in trees during the summer as well as overcrowding amongst aphids which they sense when they bump into each other too often. The heteroecious life cycle (which is mainly linked to consumption of angiosperms and represents 10% of all aphids) is believed to have evolved from the ancestral autoecious form (on conifers); this is believed to have reverted to the ancestral form in some species that were once heteroecious.

Four types of alate (winged) aphid morphs exist, known as polymorphisms

• Emigrants (heteroecious only). They are produced on primary host and migrate to secondary host, this is once again due to quality of food decreasing AND to a lesser extent overcrowding. These aphids are capable of eating off of both hosts.
• Alate exules (produced on secondary host if heteroecious, if autoecious will be produced on host anyway). For the alate exules the same factors apply as for emigrants EXCEPT that crowding is more important.
• Gynoparae (heteroecious only and produced on secondary host in response to longer nights and falling temperature).Nymphs can only feed on secondary host, they are unable to consume the primary host,
• Males (produced on secondary host in heteroecious, in autoecious, normal host). These too are produced in response to longer nights and decreased temperature.

Of these, only 0.6% of autumn alate migrants will find host plants i.e. gynoparae.

Why do aphids host alternate?

• Nutritional optimization (right)
• Temperature tolerance – morphs adapted to part temperature.
• Oviposition and rendezvous sites
• Induced host-plant defences - plants abscise galled tissue; evidence shows that some plants selectively drop galled leaves earlier than ungalled ones.
• Increasing chance of new clones produced

  Autoecious (increase likelihood to meet same individual

  Heteroecious (decreases chance of meeting self therefore mating with different clone). Better oviposition sites on trees than herbaceous plants as dies and rots in winter. Problem: survival rate of autoecious vs heteroecious is similar.

• Enemy escape – same tree all year round increases risk of predator finding you, also you can’t escape, however if you move to a secondary host, you can circumvent them for a period of time. One of the problems with this is that on species of primary host, you will find the autoecious aphid that will continue feeding the predator.
• Fundatrix specialisation – host alternation a constraint imposed by specialised feeding requirements of the fundatrix morph as heteroecious life cycle is not the optimal one.
• Many host alternating species are the biggest aphid pests:

Aphis fabae

Metopolophium dirhodum

Myzus persicae

Rhopalosiphum padi