MASSES of stuff deleted.
> : Richard Lee <rrcolley at uclink2.berkeley.edu> writes:
> : Mitochondria is [sic] indeed passed solely through the line of
matriarchy,
> Yep, but the reason for this is _spooky!_ The mitochondrial DNA from
the
> male is contained in the tail of the sperm, which breaks off before
it
> can contribute its mitochondrial DNA. We are _truly_ our mothers'
> children, i.e., moms contribute _slightly_ more than 50% to the
child's
> makeup.
Wrong, sorry. The sperm *midpiece* contains about 30-50 mitochondria
and in most mammals is indeed incorporated at fertilization. In fact
the sperm tail centrosomal material is the template (probably not
absolutely essential) for the spindle apparatus of the first mitotic
division. However, the egg contains redundant mechanisms that can form
a spindle in the absence of sperm cytoplasm, as seen in parthenogenetic
organisms (many lizards) and in experimental situations where embryos
and live young have been produced by fusion of isolated roudn spermatid
(haploid) nuclei with eggs. The first pregnancies have been announced
in humans by this technique (references supplied if anyone's
interested).
The jury is out as to whether maternal inheritance is due to swamping
(1000 fold ratio of maternal to paternal mtDNA), degradation of sperm
mtDNA (several weeks of independent activity may have worn it out), or
selective destruction or inactivation. Some evidence supports the
latter, as there is ultrastructural evidence of breakdown of the sperm
mitochondria.
A wide survey of organisms reveals that maternal inheritance, while
widespread, is not universal. It's paternal in conifers, for example.
Biparental inheritance is extremely rare (some mussels). This has led
Hurst to suggest that, in multicellular organisms with multiple genome
inheritance, natural selection has produced a situation wherein one
parent's cytoplasmic genes are invariably suppressed to avoid lethal
"genomic conflict" (ie the cytoplasmic genes are subservient to the
greater interest of the somatic genes and the survival of the germ cell
lineage).
Hurst, L.D. (1992). Intragenomic conflict as an evolutionary force. Proc
Roy Soc Lond B., 248, 135-140.
Ogura, A., Matsuda, J., and Yanagimachi, R. (1994). Birth of normal
young after electrofusion of mouse oocytes with round spermatids. Proc
Natl Acad Sci U S A., 91, 7460-7462.
Ogura, A., Yanagimachi, R., and Usui, N. (1993). Behaviour of hamster
and mouse round spermatid nuclei incorporated into mature oocytes by
electrofusion. Zygote., 1, 1-8.
Schatten, G. (1994). The centrosome and its mode of inheritance - the
reduction of the centrosome during gametogenesis and its restoration
during fertilization. Dev Biol., 163, 534.
Sofikitis, N., Miyagawa, I., Sharlip, I., Hellstrom, W., Mekras, G., and
Mastelou, E. (1995). Human pregnancies achieved by intra-ooplasmic
injections of round spermatid (RS) nuclei isolated from testicular
tissue of azoospermic men. Las Vegas, Nevada: AUA Meeting
Abstracts/PRISM Productions, 0616.
Yanagida, K., Bedford, J.M., and Yanagimachi, R. (1991). Cleavage of
rabbit eggs after microsurgical injection of testicular spermatozoa. Hum
Reprod., 6, 277-279.
