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
Megalospheric
Gamontogamy
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