Mammal chromosomes are all diploid except for the X - Y sex cells, which are haploid. Plants evolved a different method of reproduction whereby the diploid generation first forms sporophytes. The sporophytes generate the haploid generation (spores) via meiosis. The spores undergo mitosis prior to the occurrence of fertilization, which is different than in humans. The origin of the haploid sex cells is a completely different subject and arose from a mutation in unicellular gametes over 2 billion years ago.
PLANTS DO IT DIFFERENTLY
In animals, the body grows to adulthood; in the adult body there are specialized locations where
Reduction Division (Meiosis) occurs to produce special cells with half the normal amount of genetic material
(chromosomes). These special cells are called gametes (sperm or egg cells) because they can directly engage
in fertilization. Fertilization involves pooling the chromosomes of the egg and the sperm to produce a
fertilized egg having once again the normal chromosome content, and thus a normal new individual results.
Plants are different! A multicellular plant body does have specialized locations where Reduction
Division produces special cells containing half the chromosomes, but these special cells are unable to engage
in fertilization themselves. Thus these special cells are called SPORES. These spores must first grow into
another multicellular structure, which then produces the sperm or egg cells that can undergo fertilization.
Thus, in a complete plant life cycle there are TWO different multicellular structures:
--the fertilized egg grows into one structure called the sporophyte which produces spores;
--the spores grow into another structure called the gametophyte which produces the gametes (sperm or egg).
The plant life cycle thus has two different alternating multicellular structures, and is therefore referred
to as “Alternation of Generations”.
C:\Documents and Settings\Kodi\My Documents\Plant Reproduction - Alt of Gen, Gymnosperms, Angiosperms.pdf
Origin of sex/haploid gametes - isogamy to oogamy:
"Isogamous sexual reproduction occurs through "plus" (MT+) and "minus" (MT-) mating types. MT- represents a "dominant sex" because a particular gene, MID ("minus-dominance") of Chlomydomonas reinhardtii [a primitive unicellular Green Algae] is both necessary and sufficient to cause the cells to differentiate as MT- isogametes. The "PlestMID" gene is present only in the male genome, and it encodes a protein localized abundantly in the nuclei of mature sperm. The findings indicate that P. starrii maleness evolved from the dominant sex (MT-) of its isogamous ancestor." Source: Nozaki, Hisayoshi, et. al. (2006) in Current Biology, December 19th issue
Composite (left) and reconstructed (right) skeletons of D. szalayi, the oldest known ancestor of primates:
They made the rare discovery of the nearly complete skeletons of two plesiadapiform species (above), now named Ignacius clarkforkensis and Dryomomys szalayi, embedded in limestone outside Yellowstone National Park. "The branches of that tree which includes humans, chimps, gorillas, baboons, and lemurs, can all be traced back 55 million years ago, when the first undisputed primates appear in the fossil record....Primates must have acquired their traits gradually, because plesiadapiforms have some, but not all, of the characteristics of primates"
http://www.msnbc.msn.com/id/17010079/
Archaeologists in Italy have discovered a couple buried in eternal embrace: still hugging 5,000 years ago:
http://www.msnbc.msn.com/id/17011786/