Insect holometabolous life cycle

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The evolution of the insect holometabolous life cycle and its role in the diversification of the class Insecta.

Introduction

* What is an insect

o Class Insecta, Habitats (nearly all!), Common features

o Emphasise v. Diverse

o 3 separate life history strategies, ametaboly, hemimetaboly and holometably, the evolution of which has been a fiercely contested problem ever since the time of Aristotle [8].

* Diversification of insects (brief overview)

* What is metamorphosis?

o Animals that undergo metamorphosis - not just insects (inc. Lampreys, eels, flatfish[22] , frogs etc - mention axolotl, neoteny)

o Many different ideas even amongst experts on what the definition of metamorphosis is.

§ Dictionary definition

§ Michael Hadfield [22] - Can be no absolute definition - lack of homology. But he always used for transition from Larva to Juvenile.

§ However in contrast Christos D. Georgiou [22] defines metamorphosis as “the whole process whereby the final organisation and pattern of the organism is established” says it is “a synonym of morphogenesis” and occurs in all eukaryotic organisms.

o Michael Hadfield says that “the most important thing is that the person using the word defines what he or she means by metamorphosis in the context he or she is writing.” [22]

Different insect life histories

* Moulting for discontinuous growth, due to rigid sclerotinized cuticle [23], JH control of type of moult, Ecdysteroids stimulate apolysis and ecdysis.

* Ametaboly

o Primative life history strategy - modern apterygota (thysanaura, archaeognatha) [23][25], other arthropods such as myriopoda and hexapoda, and probably in all primitive pterygota [24].

o Continuous development from egg to adult [24] Hatchling emerges from egg as miniature adult only without genitalia [23] proceeds through numerous immature stages increasing in size until final nymph/adult moult [8]

o Indeterminate growth - moult throughout life even after sexual maturity [8]

o As no difference in habitat/food = competition between immature stages and adult

* Hemimetaboly

o Partial/incomplete metamorphosis, no pupal stage [25]

o Palroptera (Ephemoptera and odonata) and Neoptera (including dermaptera, isopteran, phasmida, hemiptera, thysanoptera and the secondarily wingless order phthiraptera)

o Growth is largely direct [8], after eclosion, undergoes several nymphal moults. Immature stages resemble adult but lack wings and genitalia except in emphemoptera, which has winged pre-adults. Wing buds grow outside cuticle at each moult (gives the name exopterygota) [8]. At final nymphal moult, gains genitalia and fully grown wings - adult [23]

o Kukalova-peck - hemimetaboly evolved several times [24]

o Polyphyletic group

o According to fossil record evolved before holometabola [8]

* Holometaboly

o Endopterygota

o Juvenile stage = larva (may include several stages)

§ Types of larva [23][3]

* Polypod

* Oligopod

* Apod

o Larval-pupal moult - Pupa

§ Types of pupa (e.g. can be mobile, can have sclerotinized puparium)

§ Attaches to leaf/twig with silk or burrows into substrate

§ Complete deconstruction and reassembly

§ Very high metabolic activity

o Coleoptera, Hymenoptera, diptera, Lepidoptera, mecoptera, siphonaptera, trichoptera.

Evolution (some theories)

(Reference - mainly [8] unless otherwise noted, I found it very difficult to get hold of the original papers for many of these ideas)

* “Second egg”/”imperfect egg” hypothesis and derivatives

o Harvey - 1651 - original 2nd egg theory

§ Holometabolous insects hatch from “imperfect eggs” - before embryonic development complete

§ Pupa = “2nd egg “ / continuation of embryonic events

o Lubbock - 1883 - Also reasoned that larval morphology subject to natural selection

o Henson - 1946 - Repetition theory (reworking of 2nd egg)

§ each moult is a continuation of embryonic development, morphogenesis suppressed in differing degrees - metamorphic moult suppressed least.

§ Basis in gut development

* De-embryonization hypothesis + derivatives

o Berlese - 1913 + Imms - 1931

§ larval stage occurred because embryogenesis events moved to posembryonic life - later occurring nymphal stage was compressed into pupal stage

§ 3 stages of embryonic development: Protopod, polypod, oligopod

§ Stage at which insect hatches, determines future life history strategy

o Heslop-harrison - 1958

§ Both strategies - modification of a primitive insect life cycle

* “Mould” theory

o Poyarkoff - 1914

§ Studied skeletal muscle - thought pupal cuticle was a “mould” for adult musculature

* Pupal stage = derived final stage nymph

o Hinton - 1959/1963

§ Premier theory for many years, originally supported Poyarkoff - couldn't prove

§ Turned to wing development to explain

Endocrine control of metamorphosis

* Explain endocrine changes during metamorphosis [11][15][18]

o Juvenile Hormone

o 20-hydroxyecdysone

o broad - pupal specifier

* importance in origin of metamorphosis theories

o Novak - 1966 [8]

o Truman and Riddiford - 1999 [2]

o Support of Lubbock/Berlese theories (broad) [8]

A radical recent theory

* Caterpillars evolved from onychophorans by hybridogenesis [1]

* Flaws of this theory [20]

The role of metamorphosis in diversification of insect taxa

* Ability to separate food source + habitat of juvenile and adult - allowed to eradicate competition between different life stages

* Can exploit more than one habitat in lifetime

Conclusions

I have some more reading to do before I can come up with a decent conclusion to this!

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