title: Doubly uniparental inheritance of mitochondria as a model system for studying germ line formation.
abstract: BACKGROUND
  Doubly Uniparental Inheritance (DUI) of mitochondria occurs when both mothers and fathers are capable of transmitting mitochondria to their offspring, in contrast to the typical Strictly Maternal Inheritance (SMI). DUI was found in some bivalve molluscs, in which two mitochondrial genomes are inherited, one through eggs, the other through sperm. During male embryo development, spermatozoon mitochondria aggregate in proximity of the first cleavage furrow and end up in the primordial germ cells, while they are dispersed in female embryos.
  
  
  RESULTS
  We used MitoTracker, microtubule staining and transmission electron microscopy to examine the mechanisms of this unusual distribution of sperm mitochondria in the DUI species Ruditapes philippinarum. Our results suggest that in male embryos the midbody deriving from the mitotic spindle of the first division concurs in positioning the aggregate of sperm mitochondria. Furthermore, an immunocytochemical analysis showed that the germ line determinant Vasa segregates close to the first cleavage furrow.
  
  
  CONCLUSIONS
  In DUI male embryos, spermatozoon mitochondria aggregate in a stable area on the animal-vegetal axis: in organisms with spiral segmentation this zone is not involved in cleavage, so the aggregation is maintained. Moreover, sperm mitochondria reach the same embryonic area in which also germ plasm is transferred. In 2-blastomere embryos, the segregation of sperm mitochondria in the same region with Vasa suggests their contribution in male germ line formation. In DUI male embryos, M-type mitochondria must be recognized by egg factors to be actively transferred in the germ line, where they become dominant replacing the Balbiani body mitochondria. The typical features of germ line assembly point to a common biological mechanism shared by DUI and SMI organisms. Although the molecular dynamics of the segregation of sperm mitochondria in DUI species are unknown, they could be a variation of the mechanism regulating the mitochondrial bottleneck in all metazoans.
authors.affiliation: Department of Biologia Evoluzionistica Sperimentale, University of Bologna, Bologna, Italy. liliana.milani@unibo.it
authors.forename: Liliana
authors.initials: L
authors.lastname: Milani
authors.forename: Fabrizio
authors.initials: F
authors.lastname: Ghiselli
authors.forename: Maria Gabriella
authors.initials: MG
authors.lastname: Maurizii
authors.forename: Marco
authors.initials: M
authors.lastname: Passamonti
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D000918:Antibody Specificity / Q000276:immunology; D049872:Bivalvia / Q000166:cytology / Q000235:genetics* / Q000648:ultrastructure; D020090:Chromosome Segregation; D053487:DEAD-box RNA Helicases / Q000737:chemistry; D004625:Embryo, Nonmammalian / Q000166:cytology / Q000378:metabolism / Q000648:ultrastructure; D005260:Female; D006066:Gonads / Q000033:anatomy & histology; D040582:Inheritance Patterns / Q000235:genetics*; D008297:Male; D008870:Microtubules / Q000378:metabolism / Q000648:ultrastructure; D008928:Mitochondria / Q000235:genetics*; D008954:Models, Biological; D008969:Molecular Sequence Data; D017434:Protein Structure, Tertiary; D021381:Protein Transport; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid; D013094:Spermatozoa / Q000166:cytology / Q000378:metabolism* / Q000648:ultrastructure
chemical_list: D053487:DEAD-box RNA Helicases