(none)

Patrick Elvander elvander at biology.ucsc.edu
Fri Nov 21 18:36:12 EST 1997


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.

_________________________________________________________
	Patrick Elvander	Department of Biology
	Lecturer in Biology	Sinsheimer Labs
	408-459-3674		University of California
				1156 High St.
elvander at biology.ucsc.edu	Santa Cruz, CA  95064
_________________________________________________________





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