**This post is a part of the series on the 2019 AGA Presidential Symposium – Sex and Asex: the genetics of complex life cycles**
About the Author: Taylor Williams wrote this post as a part of Dr. Stacy Krueger-Hadfield’s Ecological Genetics course taken as a special topics course at the College of Charleston. She is a MS student in Dr. Heather Spalding’s lab where she is studying the reproductive state and population connectivity of a cryptogenic alga in the Northwestern Hawaiian Islands. Taylor received her BS in Marine Biology from the University of Hawaiʻi, Mānoa.
Understanding the contribution that ecological variation plays on the predominance of sex is one of evolutionary biologist’s greatest questions.
Sexuality and asexuality each present their own challenges; sexuality has the costs of producing males and finding a mate along with the cost of recombination while asexuality has the cost of clonality and accumulation of bad mutations. Many asexuals are also the result of hybridization which can lead to lower fitness in F1 progeny. It’s challenging to link negative fitness effects to hybridization directly because it is difficult to differentiate the effects of hybridization from the effects of asexuality. Self-fertilization and outbreeding depression can also reduce fitness which makes assessing the effects of hybridization even more challenging.
In the Sex/Asex special issue, Rushworth & Mitchell-Olds (2020) explore fitness consequences of hybridization in controlled conditions using wildflowers.
Boechera (Figure 1) is highly self-fertilizing when reproducing sexually but hybridization is also common in the wild. This makes them a model organism for assessing the link between reproductive mode and hybridization.
A series of greenhouse crosses of B. stricta and B. retrofracta were used to assess the link between hybridization and asexuality to identify the source of reduced hybrid fitness. All possible crosses of the wild-collected plants were conducted, and individuals were allowed to self-fertilize simultaneously. Fruiting success, viability, and sterility of the F1 offspring was calculated and then F1 crosses were conducted. The fitness of the F2 offspring was assessed and GLMs provided insight into how cross type (hybrid or selfed) influenced their fitness components.
Total fitness was lower in the hybrid F2 individuals than the selfed lineages. Genotype accounted for 14% of the F2 generation’s increased fitness suggesting genotype plays a large role in the ultimate trajectory of hybrids. Nevertheless, the success of hybrids was rare and directionality of the cross was found to have profound effects on success and fitness. B. stricta as the maternal parent resulted in 23.9% more success at producing fruit; this trend was also observed in wild populations. Hybridization negatively impacts some fitness components of sexual lineages via outbreeding depression, reproductive isolation, and gamete loss, while others are improved.
The authors determined that there are scales of fitness regarding reproductive mode. Wild asexual hybrids were the most fit because they have increased overwinter survival, inbred sexual lines were the least fit, and heterozygous sexual lines fell in the middle because of the role heterosis plays.
Nearly all wild collected Boechera hybrids were asexual which contradicts what was observed in the greenhouse experiments; this shows that all apomicts are hybrids but not all hybrids are apomictic. Hybridization is independent of apomixis. In later generations, only sexual hybrids were produced which could indicate that the transition to asexuality happens rapidly and is caused by environmental factors that were absent from the study. Low fitness hybrids frequently form and die which could be why asexuality only occurs in high fitness hybrids.
Hybridization and polyploidy lead to dramatic phenotypic change that ultimately influences fitness and patterns of selection. Because all of the individuals in this study reproduced sexually the investigators were able to assess fitness effects of hybridization and outcrossing separate from asexuality and polyploidy. Fitness costs of hybrid formation due to reproductive isolation were substantial and multiple reproductively isolating barriers reduced hybrid fitness which has consequences for sexual/asexual dynamics in flowering plants.
More and more evidence is being unearthed showing the ways in which hybridization and the subsequent reduced fitness is entangled with the inherent costs of sex in the wild.
Rushworth, C. A. & Mitchell-Olds, T. (2020). The evolution of sex is tempered by costly hybridization in Boechera(Rock Cress). Journal of Heredity. 112(1), 67-77