Research Team - PD Dr. Wolfgang Staiber

Analysis of the structure and function of the germ line-limited chromosomes and of their behavior during elimination mitoses in the midge Acricotopus lucidus

The elimination of chromatin or whole chromosomes from the future somatic nuclei during germ line-soma differentiation in early embryogenesis is a genetic phenomenon found in a wide variety of animal species. Less is known about the origin, structure, and function of the additional chromosomes, which are restricted to the germ line.

In the midge Acricotopus lucidus fluorescence in situ hybridization (FISH) with labeled soma DNA to these germ line limited-chromosomes (=Ks, K is derived from ‘Keimbahn’) and soma chromosomes (=Ss) of spermato­gonial mitoses revealed that each of the nine different K types possesses large S-homologous sections. Painting probes of the three Ss were generated by micro­dissection of polytene salivary gland chromosomes and sub­sequent amplification by the degenerate oligonucleotide-primed polymerase chain reaction (DOP-PCR). Multicolor FISH demonstrated that each of the Ks, with the exception of one K type, is derived from a specific S (1). One function of the S-homologous K sections is thought to be determination of the regular occurrence of crossover events, with the resul­ting chiasmata in these sections ensuring correct segregation of the K homologs during meiosis. Reverse chromosome painting produces conclusive evidence for the hypothesis that during evolution the Ks have developed from the Ss by endopolyploidization and re­arrangements followed by the accumulation of germ line-specific repetitive DNA sequen­ces in the centro­meric regions.

In Acricotopus the Ks and the Ss pass through a complex chromosome cycle, which ex­hibits three genetic specialities:

1. The complete elimination of the Ks from the prospective somatic nuclei during early cleavage divisions (=soma elimination) (3).
2. The elimination of about half of the Ks during the first gonial mitoses of the primordial germ cells in newly hatched larvae (=germ line elimination) (2).
3. A compensating duplication of the Ks as a result of the last gonial mitosis prior to meio­sis in the young fourth instar larvae. In this mitosis all Ks move as unseparated sister chromatids to only one pole, while the Ss segregate equally (=differential mitosis) (4).

This special mitosis produces daughter cells with different chromosome constitutions and diverse developmental fates. The cells receiving all the Ks and two S sets develop into oocytes and into primary spermato­cytes, whereas the cells receiving only two S sets diffe­rentiate into a nurse cell and an aberrant spermatocyte.

In differential mitosis a preferential segregation of mitochondria occurs to one pole asso­cia­ted with an asymmetric formation of the mitotic spindle. This has been detected in living gonial cells in both sexes by using MitoTracker probes and fluorochrome-labelled paclitaxel (taxol). In males, the resulting unequal partitioning of mitochondria to the daugh­ter cells is equalised by the transport of mitochondria through a permanent cyto­plas­mic bridge from the aberrant spermatocyte to the primary spermatocyte (4). This asym­metry in the distribution and in the segregation of cytoplasmic components in differential gonial mitosis in Acricotopus may be involved in the process of cell-fate determination.

selected publications:

1. Staiber, W., Schiffkowski, C. (2000). Structural evolution of the germ line-limited chromo­somes in Acricotopus. Chromosoma 109, 343-349.

2. Staiber, W. (2005).Cytogenetic analysis of partial elimination of germ line limited chromosomes from primary germ cells in Acricotopus lucidus (Diptera, Chirono­mi­dae). Cytologia 70, 7-12.

3. Staiber, W. (2006). Chromosome elimination in germ line - soma differentiation of Acricotopus lucidus (Diptera, Chironomidae). Genome 49, 269-274.  

4. Staiber, W. (2007). Asymmetric distribution of mitochondria and of spindle microtubules in opposite directions in differential mitosis of germ line cells in Acricotopus. Cell Tiss. Res. 329, 197–203