Molecular Genetics

Facts

Chlamydomonas rheinhardtii exists in two mating types, "+" and "-". Each cell contains a single large plastid. After fusion of + and - cells, the plastid DNA of the - plastid is destroyed. Destruction requires a single gene present in the + nucleus. An X. laevis egg cell contains about 108 molecules of mitochondrial DNA. An X.laevis sperm cell contains about 102 molecules of mtDNA.

Interpretations

Causes of uniparental inheritance vary:

• Relative volumes of egg and sperm cytoplasm may play a role.
• Organelles may be excluded during the formation of male gametes.
• Sperm may inject their nuclei into egg cells, rather than fusing with them.
• There can be, as occurs in C. reinhardtii, active selective destruction of the organelle DNA of one parent

Further information

• Differential DNA methylation of C. reinhardtii plastid DNA is thought to serve as the tag differentiating plastids of the two mating types (ref).
• Consequences of uniparental inheritance for evolution are being explored.
• In interspecific mating studeis paternal mitochondrial DNA was detected in some mouse embryos and in over half the neonates. Such paternal DNA randomly survives in only a few tissues (ref). In intraspecific crosses, paternal DNA disappeared from all embryos during embryogenesis, implying an active species specific destruction mechanism (ref).

This is page 1373 of Molecular Genetics by Ulrich Melcher, © 1997, 1998, 2001, 2003