Chromosomal study of a lamprey (Lampetra zanandreai Vladykov, 1955) (Petromyzonida: Petromyzontiformes): conventional and FISH analysis

Abstract

Karyotype and other chromosomal characteristics in the Adriatic brook lamprey Lampetra zanandreai, representative of one of the most ancestral group of vertebrates, were examined using conventional (Ag-staining, C-banding as well as CMA(3) and DAPI fluorescence) and molecular (FISH with 18/28S rDNA and EcoRI satDNA as probes) protocols with metaphase chromosomes derived from whole blood cultures. The chromosome complement had a modal diploid chromosome number of 2n = 164, as in other petromyzontid lamprey species. Ag-staining and CMA(3) fluorescence, as well as FISH with 18/28S rDNA probes, detected nucleolar organizer regions (NORs) close to the centromeres of the biarmed chromosomes of pairs 1 and 2, the largest chromosome pairs of the complement. In addition to NORs, CMA(3) fluorescence revealed positive signals in approximately 40 other chromosomes. DAPI stained mostly centromeric regions of many chromosomes as well as conspicuously massive blocks overlapping NOR sites. C-banding evidenced a large amount of constitutive heterochromatin in somatic chromosomes, with approximately 40 C-positive acrocentric elements completely heterochromatic, corresponding with the 40 CMA(3)+ chromosomes and positive heterochromatic blocks in pericentromeric regions of chromosome pairs 1 and 2. Polymerase chain reaction (PCR)-based cloning of satellite DNA with primers derived from Petromyzon marinus centromeric sequences was successful for L. zanandreai genomic DNA. The sequence was AT-rich (59%) and characterized by short consensus motifs similar to other centromeric satellite motifs. FISH using satDNA clones as a probe produced a fluorescent signal on a single pair of small chromosomes. This sequence was PCR-amplified also in L. planeri and P. marinus genomic DNA, and the evolution of this repetitive element in the above species was analysed.

Genome Duplication in Early Vertebrates: Insights from Agnathan Cytogenetics

“…A striking difference in the level of GC-content was also reported with high GC 4 in lampreys (70-90%) and only moderate values (40-60%) in hagfishes [Kuraku and Kuratani, 2006]. The nonoverlapping genome sizes of the direct-developing hagfishes (4.6-9.2 pg/n) and the metamorphosing lampreys (1.3-4.2 pg/n) could represent an example of genome size constraint linked to developmental complexity [Hardie and Hebert, 2004;Gregory, 2005;Arai, 2011;Caputo et al, 2011]. A possible similarity in agnathan genome biology concerns the programmed rearrangement during early embryogenesis, leading to the loss of approximately 20% of the germline DNA (including transcribed genes) in lampreys and to the elimination of entire chromosomes enriched in highly repetitive sequences in hagfishes (over 80% of the germline DNA; see below).…”

“…The geotriine and petromyzonine species are characterized by chromosome complements dominated by the presence of acrocentric or 'dot-like' chromosomes. In the petromyzonine species for which a more resolved karyotype was obtained, 2 biarmed pairs are the largest elements of the karyotype [Robinson et al, 1974;Caputo et al, 2011]. A modal number of 164 chromosomes was observed in many petromyzonine genera, like Lampetra [Howell and Denton, 1969;Robinson et al, 1974;Potter and Robinson, 1981;Caputo et al, 2011], Ichtyomyzon [Howell and Duckett, 1971] and Petromyzon [Smith et al, 2010a], thus documenting a widespread distribution of this diploid chromosome number in boreal lampreys.…”

“…Active NOR location was studied by silver staining only in 4 petromyzonines (3 species of Lethenteron [Suzuki et al, 1999] and 1 of Lampetra [Caputo et al, 2011]), in which they are on the largest biarmed chromosome pair, in a pericentromeric position. In Lampetra , no additional NORs were detected, when the investigation was carried by CMA 3 staining and FISH with ribosomal probes [Caputo et al, 2011].…”

“…Active NOR location was studied by silver staining only in 4 petromyzonines (3 species of Lethenteron [Suzuki et al, 1999] and 1 of Lampetra [Caputo et al, 2011]), in which they are on the largest biarmed chromosome pair, in a pericentromeric position. In Lampetra , no additional NORs were detected, when the investigation was carried by CMA 3 staining and FISH with ribosomal probes [Caputo et al, 2011]. Interestingly, correspondence of GC-rich DNA with rDNA clusters in this representative of one of the most ancient group of vertebrates seems to indicate that the association of GCrich DNA with NOR sites is the ancestral condition for vertebrates.…”

“…Studies on heterochromatin distribution in lamprey chromosomes were only accomplished for a petromyzonine species, L. zanandreai [Caputo et al, 2011]. Despite the small genome size of this species (1.40 pg/n), a large amount of constitutive heterochromatin was evidenced by C-banding in somatic chromosomes, as approximately 40 acrocentric elements were C-positive as well as large centromeric and pericentromeric regions of the largest biarmed chromosomes ( fig.…”