Genetic causes and genomic consequences of breakdown of distyly in Linum trigynum.

Distyly is an iconic floral polymorphism governed by a supergene, which promotes efficient pollen transfer and outcrossing through reciprocal differences in the position of sexual organs in flowers, often coupled with heteromorphic self-incompatibility (SI). Distyly has evolved convergently in multiple flowering plant lineages, but has also broken down repeatedly, often resulting in homostylous, self-compatible populations with elevated rates of self-fertilization. Here, we aimed to study the genetic causes and genomic consequences of the shift to homostyly in Linum trigynum, which is closely related to distylous Linum tenue. Building on a high-quality genome assembly, we show that L. trigynum harbors a genomic region homologous to the dominant haplotype of the distyly supergene conferring long stamens and short styles in L. tenue, suggesting that loss of distyly first occurred in a short-styled individual. In contrast to homostylous Primula and Fagopyrum, L. trigynum harbors no fixed loss-of-function mutations in coding sequences of S-linked distyly candidate genes. Instead, floral gene expression analyses and controlled crosses suggest that mutations downregulating the S-linked LtWDR-44 candidate gene for male SI and/or anther height could underlie homostyly and self-compatibility (SC) in L. trigynum. Population genomic analyses of 224 whole-genome sequences further demonstrate that L. trigynum is highly self-fertilizing, exhibits significantly lower genetic diversity genome-wide, and is experiencing relaxed purifying selection and less frequent positive selection on nonsynonymous mutations relative to L. tenue. Our analyses shed light on the loss of distyly in L. trigynum, and advance our understanding of a common evolutionary transition in flowering plants.

Genomic analyses of the Linum distyly supergene reveal convergent evolution at the molecular level.

Supergenes govern multi-trait-balanced polymorphisms in a wide range of systems; however, our understanding of their origins and evolution remains incomplete. The reciprocal placement of stigmas and anthers in pin and thrum floral morphs of distylous species constitutes an iconic example of a balanced polymorphism governed by a supergene, the distyly S-locus. Recent studies have shown that the Primula and Turnera distyly supergenes are both hemizygous in thrums, but it remains unknown whether hemizygosity is pervasive among distyly S-loci. As hemizygosity has major consequences for supergene evolution and loss, clarifying whether this genetic architecture is shared among distylous species is critical. Here, we have characterized the genetic architecture and evolution of the distyly supergene in Linum by generating a chromosome-level genome assembly of Linum tenue, followed by the identification of the S-locus using population genomic data. We show that hemizygosity and thrum-specific expression of S-linked genes, including a pistil-expressed candidate gene for style length, are major features of the Linum S-locus. Structural variation is likely instrumental for recombination suppression, and although the non-recombining dominant haplotype has accumulated transposable elements, S-linked genes are not under relaxed purifying selection. Our findings reveal remarkable convergence in the genetic architecture and evolution of independently derived distyly supergenes, provide a counterexample to classic inversion-based supergenes, and shed new light on the origin and maintenance of an iconic floral polymorphism.

The Genomic Architecture and Evolutionary Fates of Supergenes.

Supergenes are genomic regions containing sets of tightly linked loci that control multi-trait phenotypic polymorphisms under balancing selection. Recent advances in genomics have uncovered significant variation in both the genomic architecture as well as the mode of origin of supergenes across diverse organismal systems. Although the role of genomic architecture for the origin of supergenes has been much discussed, differences in the genomic architecture also subsequently affect the evolutionary trajectory of supergenes and the rate of degeneration of supergene haplotypes. In this review, we synthesize recent genomic work and historical models of supergene evolution, highlighting how the genomic architecture of supergenes affects their evolutionary fate. We discuss how recent findings on classic supergenes involved in governing ant colony social form, mimicry in butterflies, and heterostyly in flowering plants relate to theoretical expectations. Furthermore, we use forward simulations to demonstrate that differences in genomic architecture affect the degeneration of supergenes. Finally, we discuss implications of the evolution of supergene haplotypes for the long-term fate of balanced polymorphisms governed by supergenes.

Minimal-Risk Seed Heteromorphism: Proportions of Seed Morphs for Optimal Risk-Averse Heteromorphic Strategies.

Seed heteromorphism is the reproductive strategy characterized by the simultaneous production of multiple seed types. While comparing heteromorphic to monomorphic strategies is mathematically simple, there is no explicit test for assessing which ratio of seed morphs minimizes fitness variance, and hence offers a basis for comparing different heteromorphic strategies. Such a test may be particularly valuable when more than two distinct morphs are present, since many strategies may have equivalent geometric fitnesses. As noted by Gillespie (1974), in these cases avoiding rare but evolutionarily important instances of severe reductions in fitness involves the minimization of variation in fitness-i.e., risk. Here I compute the optimal proportions of two or more seed morphs for heteromorphic strategies that either: (1) minimize total fitness variance; or (2) maximize the fitness-risk ratio-i.e., the "extra" fitness accrued per unit of "extra" fitness variance. This work thereby provides a testable null hypothesis to estimate the optimal frequencies of seed morphs when multiple heteromorphic strategies have evolved in environments with severe fitness risks. Moreover, it also permits the calculation of expected seed morph frequencies when more than two seed morphs are produced.

Development of polymorphic microsatellite markers for Indian tobacco, Lobelia inflata (Campanulaceae)(1.).

PREMISE OF THE STUDY: Nuclear microsatellite markers were developed for Lobelia inflata (Campanulaceae), an obligately self-fertilizing plant species, for use in the study of temporal fluctuation in allele frequency and of the genetic structure within and among populations. • METHODS AND RESULTS: We developed 28 primer pairs for L. inflata, all of which amplify CT dinucleotide repeats. We evaluated amplification of these loci in 53 L. inflata individuals at three sites in eastern North America and found that 24 loci showed microsatellite polymorphism. We also found that 16 loci amplified successfully in L. cardinalis, and 11 amplified successfully in L. siphilitica. • CONCLUSIONS: These primers will be useful for assessing allelic diversity within and among populations of L. inflata, and show potential for use in congeneric species.

Changing reproductive effort within a semelparous reproductive episode.

UNLABELLED: • PREMISE OF THE STUDY: Life-history theory predicts a trade-off between current and future reproduction for iteroparous organisms-as individuals age, the expected value of future reproduction declines, and thus reproductive effort is expected to be higher in later clutches than in earlier. In contrast, models explaining the evolution of semelparity treat semelparous reproduction as instantaneous, with no scope for intraindividual variation. However, semelparous reproduction is also extended, but over shorter time scales; whether there are similar age- or stage-specific changes in reproductive effort within a semelparous episode is unclear. In this study, we assessed whether semelparous individuals increase reproductive effort as residual reproductive value declines by comparing the reproductive phenotype of flowers at five different floral positions along a main inflorescence.• METHODS: Using the herbaceous monocarp Lobelia inflata, we conducted a longitudinal study of 409 individuals including both laboratory and field populations over three seasons. We recorded six reproductive traits-including the length of three phenological intervals as well as fruit size, seed size, and seed number-for all plants across floral positions produced throughout the reproductive episode.• KEY RESULTS: We found that while the rate of flower initiation did not change, flowers at distal (late) floral positions developed more quickly and contained larger seed than flowers at basal (early) floral positions did.• CONCLUSIONS: Our results were consistent with the hypothesis that, like iteroparous organisms, L. inflata increases reproductive effort in response to low residual reproductive value.

The continuum between semelparity and iteroparity: plastic expression of parity in response to season length manipulation in Lobelia inflata.

BACKGROUND: Semelparity and iteroparity are considered to be distinct and alternative life-history strategies, where semelparity is characterized by a single, fatal reproductive episode, and iteroparity by repeated reproduction throughout life. However, semelparous organisms do not reproduce instantaneously; typically reproduction occurs over an extended time period. If variation in reproductive allocation exists within such a prolonged reproductive episode, semelparity may be considered iteroparity over a shorter time scale.This continuity hypothesis predicts that "semelparous" organisms with relatively low probability of survival after age at first reproduction will exhibit more extreme semelparity than those with high probability of adult survival. This contrasts with the conception of semelparity as a distinct reproductive strategy expressing a discrete, single, bout of reproduction, where reproductive phenotype is expected to be relatively invariant. Here, we manipulate expected season length--and thus expected adult survival--to ask whether Lobelia inflata, a classic "semelparous" plant, exhibits plasticity along a semelparous-iteroparous continuum. RESULTS: Groups of replicated genotypes were manipulated to initiate reproduction at different points in the growing season in each of three years. In lab and field populations alike, the norm of reaction in parity across a season was as predicted by the continuity hypothesis: as individuals bolted later, they showed shorter time to, and smaller size at first reproduction, and multiplied their reproductive organs through branching, thus producing offspring more simultaneously. CONCLUSIONS: This work demonstrates that reproductive effort occurs along a semelparous-iteroparous continuum within a "semelparous" organism, and that variation in parity occurs within populations as a result of phenotypic plasticity.

Inquiry-based training improves teaching effectiveness of biology teaching assistants.

Graduate teaching assistants (GTAs) are used extensively as undergraduate science lab instructors at universities, yet they often have having minimal instructional training and little is known about effective training methods. This blind randomized control trial study assessed the impact of two training regimens on GTA teaching effectiveness. GTAs teaching undergraduate biology labs (n = 52) completed five hours of training in either inquiry-based learning pedagogy or general instructional "best practices". GTA teaching effectiveness was evaluated using: (1) a nine-factor student evaluation of educational quality; (2) a six-factor questionnaire for student learning; and (3) course grades. Ratings from both GTAs and undergraduates indicated that indicated that the inquiry-based learning pedagogy training has a positive effect on GTA teaching effectiveness.