Behavioral and spermatogenic hybrid male breakdown in Nasonia

Abstract

Several reproductive barriers exist within the Nasonia species complex, including allopatry, premating behavioral isolation, postzygotic inviability and Wolbachia-induced cytoplasmic incompatibility. Here we show that hybrid males suffer two additional reproductive disadvantages, an inability to properly court females and decreased sperm production. Hybrid behavioral sterility, characterized by a reduced ability of hybrids to perform necessary courtship behaviors, occurs in hybrids between two species of Nasonia. Hybrid males produced in crosses between N. vitripennis and N. giraulti courted females at a reduced frequency (23-69%), compared with wild-type N. vitripennis and N. giraulti males (>93%). Reduced courtship frequency was not a simple function of inactivity among hybrids. A strong effect of cytoplasmic (mitochondrial) background was also found in N. vitripennis and N. giraulti crosses; F2 hybrids with giraulti cytoplasm showing reduced ability at most stages of courtship. Hybrids produced between a younger species pair, N. giraulti and N. longicornis, were behaviorally fertile. All males possessed motile sperm, but sperm production is greatly reduced in hybrids between the older species pair, N. vitripennis and N. giraulti. This effect on hybrid males, lowered sperm counts rather than nonfunctional sperm, is different from most described cases of hybrid male sterility, and may represent an earlier stage of hybrid sperm breakdown. The results add to previous studies of F2 hybrid inviability and behavioral sterility, and indicate that Wolbachia-induced hybrid incompatibility has arisen early in species divergence, relative to behavioral sterility and spermatogenic infertility.

Figure 1

Proportions of males of each class that performed each of the six stages of courtship behaviour is plotted, with the sequential order of courtship stages along the X-axis. (A) Older species pair (vitripennis-giraulti) using giraulti females (left) and vitripennis females (right). (B) Younger species (giraulti-longicornis) pairing giraulti females (left) and longicornis females (right). Courtship behaviours shown (bottom) and are described in Table 1. Standard deviations for proportions were obtained by transforming proportions into angles (theta) via the arcsine square-root transformation. Variances of the angles were then calculated and used to determine standard deviations of theta. Standard deviations were then back-transformed to give values in proportions for visual presentation. Error bars are one standard deviation.

Figure 2

Average total sperm production per male in parental and F2 hybrid males. The phylogenetic relationship between Nasonia species is shown (top). Average sperm production per 2-3 day old male is shown (bottom). Genotypes are given as maternal/paternal[cytoplasm]. Shaded bars are parental strains, white bars are hybrid crosses. (*) significant difference from vitripennis (‡) significant difference from giraulti(Δ) significant difference from longicornis. Error bars are SEM.

Figure 3

Testes (left) and seminal vesicles (right) from parental strains (A-D) and F2 hybrids (E-F). (A - B) N. vitripennis (C - D) N. giraulti. (E – F) vitripennis-giraulti F2 [v]. (G – H) vitripennis-giraulti F2 [g]. arrows indicate nuclei bundle from late stage spermatocysts. Asterisks indicate apical end of testis. Scale bar = 20 μl.