Two Homogametic Genotypes - One Crayfish: On the Consequences of Intersexuality

Abstract

In the Australian redclaw crayfish, Cherax quadricarinatus (WZ/ZZ system), intersexuals, although exhibiting both male and female gonopores, are functional males bearing a female genotype (WZ males). Therefore, the occurrence of the unusual homogametic WW females in nature is plausible. We developed W/Z genomic sex markers and used them to investigate the genotypic structure of experimental and native C. quadricarinatus populations in Australia. We discovered, for the first time, the natural occurrence of WW females in crustacean populations. By modeling population dynamics, we found that intersexuals contribute to the growth rate of crayfish populations in the short term. Given the vastly fragmented C. quadricarinatus habitat, which is characterized by drought-flood cycles, we speculate that intersexuals contribute to the fitness of this species since they lead to occasional increment in the population growth rate which potentially supports crayfish population restoration and establishment under extinction threats or colonization events.

Keywords: evolutionary biology; genetics; genotyping; zoology.

Graphical abstract

Figure 1

Crosses of (A) male and female and (B) intersex and female crayfish. The resulting genotype and phenotype ratios in each progeny are shown in the figure.

Figure 2

Genital Openings (Gonopores) in C. quadricarinatus Male – two gonopores at the base of the fifth pereiopod (left). Female – two gonopores at the base of the third pereiopod (right). Intersex – seven combinations of male and female gonopores observed and reported by Parnes et al., in 2003 (middle).

Figure 3

Identification of the Genotype of C. quadricarinatus Using Genomic Sex Markers DNA that was extracted from 4 animals of each genotype (ZZ, WZ, WW) was used as a template to amplify sex chromosome–specific markers (Z–top, W–bottom). A 100 bp DNA ladder is given in the left part of the gels.

Figure 4

The Genotype of C. quadricarinatus Intersexuals Genomic sex markers were used to test the genotype of each intersex with every possible combination of female and male gonopores (right). The new type of intersexual that we found in this study, with one male and one female gonopore on the same side, is marked out with a red frame. ZZ male, WZ female, and WW female are given as controls. A 100 bp DNA ladder is given in the left part of the gels.

Figure 5

Sampling Locations in QLD, Australia (A–E) (A) Ironbark Redclaw Crayfish Farm, (B) Freshwater Australian Crayfish Traders Pty Ltd., (C) Klaus Cazzonelli Redclaw Farm, (D) AquaVerde Redclaw Hatchery & Farm, (E) Cherax Park Aquaculture. The map was created online through http://www.pinmaps.net.

Figure 6

Simulated Dynamics of C. quadricarinatus Population Structure (A) Finite growth rate (rate of change in a generation) of crayfish populations as a function of time. Green line: a population with α = 4% (fraction of WZ females emerging as intersex IS) and the initial fraction of intersexuals in the founder population (i.e., at time t = 0) larger than that at the long-term stable distribution (LTSD) (IS₀ = 50% > IS_∞ and WZ₀ = 50%, with ZZ₀ = WW₀ = 0); red line: same α as above but with the initial fraction of intersexuals in the founder population lower than that at LTSD (ZZ₀ = 50% and WZ₀ = 50%, with IS₀ = WW₀ = 0 < IS_∞); black and gray lines: finite growth rate for two Australian native crayfish populations as described in Table 3 with an initial structure of N₀ = [ZZ₀ = 423, IS₀ = 10, WZ₀ = 417, WW₀ = 2] and N₀ = [ZZ₀ = 276, IS₀ = 7, WZ₀ = 284, WW₀ = 3] for AU-A and AU-B, respectively, and α = 4%; blue line: a population with a binary sex structure (no WZ progeny emerging as intersexuals, i.e., α = 0, only ZZ and WZ individuals). (B) Trajectories of population size in time for α = 4% and an initial fraction of intersexuals that is larger than that at the LTSD [IS₀ = 100 (50%)>IS_∞ and WZ₀ = 100 (50%), with ZZ₀ = WW₀ = 0]. After an initial transitory period, depending upon the initial conditions, the population reaches a stable structure (i.e., a constant fraction of individuals in each class) while growing exponentially (long-term Malthusian growth in the semi-logarithmic diagram is represented by a straight line). (C) As in (B) but with an initial fraction of intersexuals that is lower than that at the LTSD [ZZ₀ = 100 (50%) and WZ₀ = 100 (50%), with IS₀ = WW₀ = 0 < IS_∞]. The vertical dashed lines in B and C represent the generation in which the population size reaches or exceeds 1,000 individuals.

Figure 7

Phylogenetic Analysis of 38 Crustacean Species The evolutionary history was inferred by using the maximum likelihood method and Jones-Taylor-Thornton (JTT) matrix-based model (Jones et al., 1992). Supporting values (bootstrapping 1000 tests) and branch length scale are given. The infraorder of each species is indicated. Species with reproductive strategies other than gonochorism are colored.