Hybridization and the Origin of Contagious Asexuality in Daphnia pulex.
Hybridization plays a potentially important role in the origin of obligate parthenogenesis (OP) in many organisms. However, it remains controversial whether hybridization directly triggers the transition from sexual reproduction to obligate asexuality or a hybrid genetic background enables asexual species to persist. Furthermore, we know little about the specific genetic elements from the divergent, yet still hybridizing lineages responsible for this transition and how these elements are further spread to create other OP lineages. In this study, we address these questions in Daphnia pulex, where cyclically parthenogenetic (CP) and OP lineages coexist. Ancestry estimates and whole-genome association mapping using 32 OP isolates suggest that a complex hybridization history between the parental species D. pulex and D. pulicaria is responsible for the introgression of a set of 647 D. pulicaria single nucleotide polymorphism alleles that show perfect association with OP. Crossing experiments using males of OP lineages and females of CP lineages strongly support a polygenic basis for OP. Single-sperm analyses show that although normal meiotic recombination occurs in the production of haploid sperm by males of OP lineages, a significant proportion of such sperm are polyploid, suggesting that the spread of asexual elements through these males (i.e., contagious asexuality) is much less efficient than previously envisioned. Although the current Daphnia genome annotation does not provide mechanistic insight into the nature of the asexuality-associated alleles, these alleles should be considered as candidates for future investigations on the genetic underpinnings of OP.
Department of Biology, Indiana University, Bloomington exusen@gmail.com.
Sen
S
Xu
Department of Biology, Indiana University, Bloomington.
Ken
K
Spitze
Department of Biology, Indiana University, Bloomington.
Matthew S
MS
Ackerman
Department of Biology, Indiana University, Bloomington.
Zhiqiang
Z
Ye
Department of Biology, Indiana University, Bloomington.
Lydia
L
Bright
School of Public and Environmental Affairs, Indiana University, Bloomington.
Nathan
N
Keith
School of Public and Environmental Affairs, Indiana University, Bloomington.
Craig E
CE
Jackson
School of Public and Environmental Affairs, Indiana University, Bloomington School of Biosciences, University of Birmingham, Birmingham, United Kingdom.
Joseph R
JR
Shaw
Department of Biology, Indiana University, Bloomington.
Michael
M
Lynch
D000483:Alleles; D000818:Animals; D002874:Chromosome Mapping; D003621:Daphnia / Q000235:genetics*; D019143:Evolution, Molecular; D005260:Female; D006238:Haploidy; D006824:Hybridization, Genetic; D008297:Male; D018895:Microsatellite Repeats; D008957:Models, Genetic; D010312:Parthenogenesis; D010802:Phylogeny; D011123:Polyploidy; D012100:Reproduction, Asexual / Q000235:genetics*
Daphnia; hybridization; introgression; obligate parthenogenesis; sperm