Heterospory

Question 1

what is the difference between homospory and heterospory?

Answer 1

• Similar plant structures (both have strobili)
• Heterospory plants differ in that sporangium have either a large number or small spores or a small number of large spores whereas homosporous plants have the same number of small spores in each sporangium

Question 2

what are three features of heterospory?

Answer 2

1. Size differentiation: megaspores and microspores
2. Unisexuality of the gametophytes that develop from each spore type (megaspores only become female megagametrophyte and vice versa)
3. Endoscopic development of gametophyte
   As gametophyte develops inside the spore, it is dependent on spore reserves (cannot photosynthesise)
   Next generation sporophytes entirely depend on unused megaspore reserves

Question 3

what is the size difference between micro and megaspores?

Answer 3

• Usually a 1000 to a few 1000 times greater volume than microspore
• Microspores are a similar size to homosporous spores

Question 4

in what groups is heterospory occuring?

Answer 4

Heterospory occurs in some lycophytes, some monilophytes and all seed plants

Question 5

when did heterospory arise and how many times?

Answer 5

• First arose about 370 million years ago in the lycophyte lineage
• Fossils of cyclostigma and bisporangiostrobus
• evolved independently multiple times (suggests it is an adaptation)

Question 6

how is heterospory the precurser of seeds?

Answer 6

• Microspores are like pollen and megaspores are like ovules of a pine
• Heterospory was a necessary precursor to the evolution of seeds

Question 7

how is heterospory adaptive?

Answer 7

• Size represents nutrient reserve for sporophyte generation and development of gametophyte
• In a shadier, more productive areas there is more competition
• Spore size increased from Silurian to Carboniferous (at the same time there was more shade and crowding developing because of increasing vasculature)
• Large spores (especially heterospory) seems to be an adaptation to novel regeneration niches created by plants themselves
• Heterospory seems to be an adaptation to shady regeneration niches

Question 8

what is the significance of the evolution of seeds?

Answer 8

seeds are what freed plants from dependence on water for reproduction