FORAMINIFERA:

PART 3 - DIMORPHISM

DIMORPHISM

• Dimorphic life cycle is complex– Alternation of sexual and asexual generations– Microspheric form– Megalospheric I form

• Trimorphism may be present– Add a megalospheric II form

• Creates taxonomic problems– Some once designated as separate species– Some were even designated as separate genera

DIMORPHISM

• Microspheric form– Larger– More chambers– Smaller proloculus– Early stages may be different

• Illustrates "ontogeny recapitulates phylogeny"

• Megalospheric form– Smaller– Fewer chambers– Larger proloculus– Early stages missing

Alternation of Generations– Duration of a cycle varies

• 1 year in tropics• 2-3 years at higher latitudes

– Cycle stages vary• Among different Foraminifera• Short circuits occur

– Microspheric (asexual or agamont or schizont) form• Diploid & Multinucleated• Reproduces asexually

– Usually during the winter– Protoplasm withdraws– Meiosis (reduction division) occurs– Haploid cells produced– Proloculus forms– Release of young megalospheric (gamont) forms

Alternation of Generations• Megalospheric (gamont) form

– One nucleus until reproduction– Matures during the summer– Withdraws protoplasm and undergoes mitosis

• Haploid, biflagellated gametes produced

• Same haploid chromosome number as parent

• Released from test of parent which empties

– Gametes fuse with gametes from another gamont• Diploid zygote (first cell) results

• Secretes a small proloculus

– Zygote develops a microspheric test (agamont or schizont)

Foram Life Cycle

• Alternation of generations

Megalospheric

Microspheric

(= agamont)

Foram Life Cycle

• Alternation of generations

Alternation of Generations

GamontogamyMegalospheric

Microspheric

Gamontogamy

Asexual Reproduction

Agamontic Asexual Reproductions May Repeat Many Times

Autogamy

Gametogamy

Dimorphism

(sexual)

What is the 3rd mode on this curve?

Dimorphism• Cornuspira sp. with embryonic coil at the

center of the microspheric form that is lacking in the megalospheric form

Dimorphism

Dimorphism

• Summary comparison between microspheric and megalospheric tests of the same species

Examples of Trimorphism

• Trimorphism:– A) Nodosaria sp.– B) Dentalina sp.– C) Marginulina sp.

• Pyrgo sp.– D) Microspheric quinqueloculine to

triloculine to biloculine stages (smallest proloculus)

– E) Megalospheric I triloculine to biloculine stages

– F) Megalospheric II biloculine stage only (largest proloculus)

END OF FILE• Vulvulina sp.• Is this microspheric or megalospheric?

• Last 2 chambers

• Middle 12 chambers

• First 4 chambers