Comparative Chromosome Painting and NOR Distribution Suggest a Complex Hybrid Origin of Triploid Lepidodactylus lugubris (Gekkonidae)

Abstract

Parthenogenesis, unisexuality and triploidy are interesting but poorly studied phenomena occurring in some reptile species. The mourning gecko (Lepidodactylus lugubris) represents a complex of diploid and triploid parthenogenetic mostly all-female populations (males occur quite rarely) widely distributed in coastal areas of the Indian and Pacific Oceans. Here, we study karyotypes of a male and two female L. lugubris (LLU) triploid individuals (3n = 66) using comparative painting with Gekko japonicus, Hemidactylus turcicus and H. platyurus chromosome specific probes to visualize the homologous regions and to reveal genus specific rearrangements. Also, we applied a 28S ribosomal DNA probe and Ag-staining to detect nucleolus organizer regions (NORs). Our results suggest that the karyotype of L. lugubris underwent a chromosome fission and a fusion after its divergence from a common ancestor of the Gekko-Hemidactylus group. The NORs were found to be located on one out of three homologs on each of LLU8, LLU15 and LLU18, thus further confirming a hybrid origin of triploid individuals. It seems that three different bisexual populations might have contributed to the origin of this triploid parthenogenetic population. We postulate that the heterozygosity in NOR localization is maintained in the triploid clone studied by the absence of recombination as described in whiptail lizards. The pattern of NOR localizations and homologous regions in males and females, as well as the absence of other detectable karyotypic differences, suggest that males arise spontaneously in all female populations and do not arise from independent hybridizations with different species.

Fig 1. Inverted DAPI-stained karyotypes of the mourning gecko (Lepidodactylus lugubris, LLU). 3n = 66.

(A) male A and (B) female A. Homologies between LLU and Hemidactylus platyurus (HPL) revealed by chromosome painting are indicated on male karyotype. Scale bar, 10 μm.

Fig 2. Example of Gekko japonicus painting probes localization onto chromosomes of Lepidodactylus lugubris.

Painting probes specific to chromosomes GJA1 (green) and GJA2 (red) each paints two triplets of LLU (female B) chromosomes: GJA1—LLU3 and LLU16; GJA2 – LLU7 and LLU13. Scale bar, 10 μm.

Fig 3. Localization of ribosomal DNA clusters in the mourning gecko (Lepidodactylus lugubris).

A partial metaphase of the LLU male A painted with human rDNA probe. Arrows indicate NORs in p-arms of two small but different in size chromosomes and in the distal part of the q-arm of a large chromosome. Scale bar, 10 μm.

Fig 4. Mapping of ribosomal RNA clusters on Lepidodactylus lugubris.

(A) Co-localization of the probe containing chromosomes GJA7+8+9 (green, paints LLU5, 8, 11, 10prox) and ribosomal probe (red) on LLU8 in female A (indicated by an arrow). (B) Co-localization of a probe containing GJA13 and GJA14 (red, paints LLU14 and LLU15) and ribosomal probe (green) on LLU15 (indicated by an arrow) in female B. (C) Co-localization of a probe containing a mixture of GJA15+16 (red, paints LLU17 and LLU18) and ribosomal probe (green) on LLU18 (indicated by an arrow) in male A. Scale bar, 10 μm.

Fig 5. AgNOR staining of Lepidodactylus lugubris chromosomes.

(A) LLU female B and (B) LLU male D. There is a polymorphism in NOR activity revealed by Ag-staining: only a single chromosome carries an active NOR in female B, but all three NORs are revealed in male D. Scale bar, 10 μm.

Fig 6. C-banding of Lepidodactylus lugubris chromosomes.

(A) female B and (B) male A. Pericentromeric and subtelomeric heterochromatic regions are dark-staining. Scale bar, 10 μm.

Fig 7. A scenario of karyotype evolution within Gekko-Lepidodactylus- Hemidactylus group.

Stars indicate chromosome fissions and circles indicate chromosome fusions. The recurrent chromosome rearrangements are underlined. The tree is based on previously published data [19].