geeta at life.cc.sunysb.edu
Sat Nov 22 11:33:59 EST 1997
Patrick Elvander (elvander at biology.ucsc.edu) wrote:
: On 21 Nov 1997, Geeta Bharathan wrote:
: > Dr. Magaly Rincon (frinconm at NEXUS.MWSU.EDU) wrote:
: > : In tetrasporic embryos (e.g., Lilium), the polar nuclei is tetraploid.
: > : During fertilization one sperm fuses with the polar nuclei and one with the
: > : egg. This means then that the endosperm is pentaploid. Is this
: > : interpretation correct?
: > Embryo-sacs (ie the female gametophyte) are tetrasporic. This means that
: > no cells walls form after either Meiosis I or II. This results in a single
: > megaspore that contains four haploid nuclei. In Lilium each of these
: > nuclei divides once mitotically, to form an 8-nucleate gametophyte
: > (=embryo-sac).
: > The two polar nuclei are each haploid, so after fusion with one haploid
: > sperm nucleus from the pollen, the endosperm nucleus that forms is
: > triploid.
: No, this is incorrect. The first writer had it right. In Lilium, which is
: tetrasporic, the four haploid nuclei resulting from meiosis are present in
: a common cell. Three migrate to one end and fuse (thus becoming triploid)
: and the remaining haploid nucleus resides at the other end (near the
: micropyle). At this point there are two nuclei in the cell (one triploid
: and one haploid) two subsequent mitoses produce four triploid nuclei and
: four haploid nuclei. One of the triploid nuclei fuses with one of the
: haploid nuclei and becomes the tetraploid fusion nucleus. When this
: nucleus fuses with one of the sperm nuclei during fertilization you get a
: pentaploid nucleus which begins the endosperm formation.
: It is confusing because all commercially available slides are of Lilium
: and this is not even the most common form of embryo sac development. Thus,
: many professors (I've done it myself!) use these slides to illustrate the
: typical development (called Polygonum type) which is closer to that
: described by the second writer above. Here it is monosporic development,
: not tetrasporic. The one surviving megaspore of the four originally
: produced goes on to develop the entire embryo sac. Thus all eight nuclei
: are haploid. Two of these haploid nuclei fuse to form a diploid fusion
: nucleus which, upon fertilization by a sperm nucleus becomes a triploid
: endosperm nucleus.
You are correct about the mode of development in Lilium, and I was wrong.
Thank you for pointing this out.
However, I do want to say that there is variation in tetrasporic
e.sacs, and that the mode I described does exist--only, not in Lilium.
I described what is called the "Adoxa" type of development; what occurs in
Lilium is the "Fritillaria" type.
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