FRST 210 lecture 12- sexual reproduction II

Homospory

• One type of spore

• Each spore germinates into a bisexual gametophyte

• Necessitates outcrossing thereby increasing the genetic diversity in offspring

What two types spores do heterosporous plants evolve to

• Megaspore → megagametophyte (egg producing)

• Microspore → microgametophyte (sperm producing)

Are all gymnosperms heterosporous?

• Yes, they are

• They bear megasporangia → megasppores) and microsporangia (→ microspores)

• have megagametophytes (egg producing) and microgametophytes (sperm producing)

Sexual reproduction in conifers (Pinophyta)

• Conifers create seed cones (sporophylls that develop megasporangia

• Conifers also develop pollen cones (sporophylls that develop microsporangia)

What do microsporophylls produce

• Microsporangia

What do megasporophylls produce

• Megasporangia

What are inside each microsporangium (pollen sac)?

• Microspore mother cells

How many microspores does one microspore mother cell produce?

• Four microspores via mitosis

• Each microspore develops into a microgametophyte (pollen grain)

When do cones develop sporangia?

• Around february-march

Why are seed cones more complex than pollen cones?

• Scales develop two ovules and a sterile bract

• An ovule contains the megasporangium that will create megaspores

• Ovules are surrounded by a protective integument

Ovuliferous scales

• Scales that bear ovules (which contain the megagametophyte) as part of a seed scale complex

• Each of these specific cone scales develops a pair of ovules, and each ovule can become a seed if fertilized

Integument

• A surrounding protective cover that also bears the megasporangium

How many megaspores do four megaspores via meiosis

• It produces four megaspores via meiosis, and only one megaspore survives and develops into a megagametophyte

When is mature pollen released and when does fertilization occur>

• ~3 weeks after pollination around April-May

• Megametophyte inside the ovule develops archegonia with eggs

• Airborne pollen are caught by a seed cone and grow a pollen tube to release sperm for fertilization

Advantages of wind pollination

• Energy efficient (Requires less energy than attracting pollinators with nectar, scent, and bright colors.

• Reliable: Wind is a more consistent pollinator than insects hose populations fluctuate

• Seed production: Wind-pollinated plants can produce large quantities of seeds

• Independence form pollinators: Wind-pollinated

Disadvantage of wind pollination

• May not be as accurate than insect/animal pollination (Most of the pollen produced by wind-pollinated plants doesn’t reach a compatible ovule.)

• The pollen produced by wind-pollinated plants is low in nutrition and spoils easily

• Hot and humid conditions, as well as rainfall, can limit the success of wind pollination

How does pollen grain fertilize an egg?

• Pollen grains arrive at the micropyle (opening of the ovule) while the functional megaspore is still developing into a megagametophyte with eggs

How many archegonia does an ovule have?

• It has one megagametophyte with 4-7 archegonia, each conitaning one identical egg cell

• Several eggs can be fertilized (polyembryony), but one develops into a mature embryo

When will cone scales disperse seeds?

• They will flex open when dry and disperse seeds for several months in fall (September-October)

When do seeds develop and enlarge

• July-August

• Integument becomes the seed coat (and seed wing)

• Megagametophyte persists and nourishes the embryo

• In gymnosperms, ovules will still develop into seeds even if they are not fertilized

When are mature seeds are released?

• In autumn

• during September-October

• Embryos enlarge and fill the seed cavity

• Megagametophyte has a firm, waxy texture

Seed cones will begin to dry out and look “woody”

Are all gymnosperm species wind pollinated?

• Most are, however a few are insect polinated

• Some species produce a sticky ‘pollination’ drop to trap pollen near the micropyle of the ovule

Do all conifers undergo sexual reproduction with cones? How long does it take for a full reproductive cycle?

• Yes, most take one full reproductive cycle (except pines that take two years)

• Cone “crops” are periodic, with large crops every 2-7 years