Apomixis for no bacteria-induced thelytoky in Diglyphus wani (Hymenoptera: Eulophidae)

Abstract

In Hymenoptera species, the reproductive mode is usually arrhenotoky, where haploid males arise from unfertilized eggs and diploid females from fertilized eggs. In addition, a few species reproduce by thelytoky, where diploid females arise from unfertilized eggs. Diploid females can be derived through various cytological mechanisms in thelytokous Hymenoptera species. Hitherto, these mechanisms were revealed mainly in endosymbiont-induced thelytokous Hymenoptera species. In contrast, thelytokous Hymenoptera species in which a reproductive manipulator has not been verified or several common endosymbionts have been excluded were paid less attention in their cytological mechanisms, for instance, Diglyphus wani (Hymenoptera: Eulophidae). Here, we investigated the cytological mechanism of D. wani using cytological methods and genetic markers. Our observations indicated that the diploid karyotypes of two strains of D. wani consist of four pairs of relatively large metacentric chromosomes and one pair of short submetacentric chromosomes (2n = 10). The arrhenotokous strains could complete normal meiosis, whereas the thelytokous strain lacked meiosis and did not expulse any polar bodies. This reproductive type of lacking meiosis is classified as apomictic thelytoky. Moreover, a total of 636 microsatellite sequences were obtained from thelytokous D. wani, dominated by dinucleotide repeats. Genetic markers results showed all three generations of offspring from thelytokous strain maintained the same genotype as their parents. Our results revealed that D. wani is the first eulophid parasitoid wasp in Hymenoptera whose thelytoky was not induced by bacteria to form an apomictic thelytoky. These findings provide a baseline for future inner molecular genetic studies of ameiotic thelytoky.

Keywords: apomictic thelytoky; cytological mechanism; heterozygosity; karyotype; microsatellite.

FIGURE 1

Different cytological mechanisms of automictic thelytoky during meiosis and their genetic effects on progeny at a locus in the event of and without recombination. The circle represents the nuclei. Vertical lines indicate chromatids, small circles connecting vertical lines indicate centromeres, and the lowercase letters indicate alleles at specific loci. The four different colored dashed lines point to four different cell fusion processes of automixis. The parent is heterozygous, indicated by ab. Homozygous progeny was indicated by aa or bb.

FIGURE 2

Karyotypes of two strains of Diglyphus wani. (A) Thelytokous female (2n = 10) (B) arrhenotokous female (2n = 10).

FIGURE 3

Chromosomes in the deposited eggs of the arrhenotokous strain. (A) Egg, creamy white or yellowish white, anterior at the top; (B) First metaphase of meiosis after oviposition; (C) sperm, found in freshly laid and sectioned eggs; (D) first anaphase of meiosis; (E) prophase of second meiosis; (F) second anaphase of meiosis and the first polar body with 5 chromosomes delayed at metaphase; (G) despiralized spermatozoon (H) the end of fertilization of female and male pronuclei (I) diploid telophase of first somatic mitosis.

FIGURE 4

Chromosomes in the deposited eggs of the thelytokous strain (A) egg, creamy white, anterior at the top; (B) first metaphase (C) first anaphase; (D) second telophase.

FIGURE 5

The number of three microsatellites types gained from six thelytokous D. wani. P1 ∼ P4 and P6 represent mono-, di-, tri-, tetra-, and hexanucleotide repeats of perfect types.