Karyological evidence of hybridogenesis in Greenlings (Teleostei: Hexagrammidae)

Abstract

Two types of natural hybrids were discovered in populations of three Hexagrammos species (Teleostei: Hexagrammidae) distributed off the southern coast of Hokkaido in the North Pacific Ocean. Both hybrids reproduce by hybridogenesis, in which the maternal haploid genome is transmitted to offspring without recombination and the paternal haploid genome is eliminated during gametogenesis. While natural hybrids are unisexual and reproduce hemiclonally by backcrossing with the paternal species (BC-P), artificial F1-hybrids between the pure species produce recombinant gametes. Thus, despite having the same genome composition, the natural hybrids and the F1-hybrids are not genetically identical. Here, to clarify the differences between both hybrids, we examined the karyotypes of the three Hexagrammos species, their natural hybrids, the artificial F1-hybrids, and several backcrosses. Artificial F1-hybrids have karyotypes and chromosome numbers that are intermediate between those of the parental species. Conversely, the natural hybrids differed from F1-hybrids by having several large metacentric chromosomes and microchromosomes. Since the entire maternal haploid genome is inherited by the natural hybrids, maternal backcrosses (BC-M) between natural hybrids and males of the maternal species (H. octogrammus; Hoc) have a hemiclonal Hoc genome with large chromosomes from the mother and a normal Hoc genome from the father. However, the large chromosomes disappear in offspring of BC-M, probably due to fissuring during gametogenesis. Similarly, microsatellite DNA analysis revealed that chromosomes of BC-M undergo recombination. These findings suggest that genetic factors associated with hemiclonal reproduction may be located on the large metacentric chromosomes of natural hybrids.

Fig 1. The lineages of specimens used in this study.

(a) artifial F₁-hybrids, (b) paternal backcross lineage, (c) maternal backcross lineage. All abbreviated species signs were referred to text. Yellow, light blue and purple of species signs indicate the H.otakii, H. octogrammus and H. aggrammus genome, respectively. Uncolored signs indicate species that not observed in this study.

Fig 2. Mitotic metaphase chromosome spread and karyotypes.

(a) H. octogrammus, (b) F₁-hybrid (Hoc × Hag), (c) F₁-hybrid (Hoc × Hot). Spreads of F₁-hybrids were showed in S1.

Fig 3. Mitotic metaphase chromosome spreads and karyotypes of natural hybrids.

(a) Hoc*/Hag type 1, (b) large metacentric chromosomes of Hoc*/Hag type 2, (c) large metacentric chromosomes and microchromosome of Hoc*/Hag type 3; (d) Hoc*/Hot type 1, (e) large metacentric chromosomes of Hoc*/Hot type 2, (f) large metacentric chromosomes and microchromosome of Hoc*/Hot type 3. Arrow heads showed large metacentric chromosomes. The remaining spreads of Hoc*/Hag and Hoc*/Hot were showed in S2 and S3, respectively.

Fig 4. Mitotic metaphase chromosome spreads and karyotypes of maternal backcross.

(a) BC-M1 ((Hoc*/Hag) × Hoc) type 1, (b) large metacentric chromosomes of BC-M1 ((Hoc*/Hag) × Hoc) type 2, (c)) large metacentric chromosomes and microchromosome of BC-M1 ((Hoc*/Hag) × Hoc) type 3; (d) BC-M2 ((Hoc*/Hot) × Hoc) type 1, (e)) large metacentric chromosomes and microchromosome of BC-M2 ((Hoc*/Hot) × Hoc) type 2. Arrow heads showed large metacentric chromosomes. The remaining spreads of BC-M1 ((Hoc*/Hag) × Hoc) and BC-M2 ((Hoc*/Hot) × Hoc) were showed in S4 and S5, respectively.

Fig 5. Mitotic metaphase chromosome spreads and karyotypes.

(a) BC-M1 × Hoc; (b) Hoc × BC-M1.

Fig 6. Schema of meiosis of natural hybrids and BC-M; (Hoc*/Hag) × Hoc.

Figures of natural hybrids were cited from a putative description of chromosome elimination proposed by Ogielska (2009) [20]. Interphase to Anaphase I is included in the first meiotic division and Anaphase II in the second meiotic division. Large chromosomes are composed of a set of two chromosomes. In natural hybrids, the large chromosome appears to be transmitted intact without recombination to the gametes. However, in BC-M, the large chromosome becomes fissured into two chromosomes after segregation and recombination of genomes from the female and the male.