Sometimes, sex is in the head
Architectural traits constrain the evolution of unisexual flowers and sexual segregation within inflorescences: an interspecific approach
Plants display an amazing diversity of reproductive strategies with and without sex. This diversity is particularly remarkable in flowering plants, as highlighted by Charles Darwin, who wrote several botanical books scrutinizing plant reproduction. One particularly influential work concerned floral variation . Darwin recognized that flowers may present different forms within a single population, with or without sex specialization. The number of species concerned is small, but they display recurrent patterns, which made it possible for Darwin to invoke natural and sexual selection to explain them. Most of early evolutionary theory on the evolution of reproductive strategies was developed in the first half of the 20th century and was based on animals. However, the pioneering work by David Lloyd from the 1970s onwards excited interest in the diversity of plant sexual strategies as models for testing adaptive hypotheses and predicting reproductive outcomes . The sex specialization of individual flowers and plants has since become one of the favorite topics of evolutionary biologists. However, attention has focused mostly on cases related to sex differentiation (dioecy and associated conditions ). Separate unisexual flower types on the same plant (monoecy and related cases, rendering the plant functionally hermaphroditic) have been much less studied, apart from their possible role in the evolution of dioecy  or their association with particular modes of pollination .
Two specific non-mutually exclusive hypotheses on the evolution of separate sexes in flowers (dicliny) have been proposed, both anchored in Lloyd’s views and Darwin’s legacy, with selfing avoidance and optimal limited resource allocation. Intermediate sex separation, in which sex morphs have different combinations of unisexual and hermaphrodite flowers, has been crucial for testing these hypotheses through comparative analyses of optimal conditions in suggested transitions. Again, cases in which floral unisexuality does not lead to sex separation have been studied much less than dioecious plants, at both the microevolutionary and macroevolutionary levels. It is surprising that the increasing availability of plant phylogenies and powerful methods for testing evolutionary transitions and correlations have not led to more studies, even though the frequency of monoecy is probably highest among diclinous species (those with unisexual flowers in any distribution among plants within a population ).
The study by Torices et al.  aims to fill this gap, offering a different perspective to that provided by Diggle & Miller  on the evolution of monoecious conditions. The authors use heads of a number of species of the sunflower family (Asteraceae) to test specifically the effect of resource limitation on the expression of sexual morphs within the head. They make use of the very particular and constant architecture of inflorescences in these species (the flower head or “capitulum”) and the diversity of sexual conditions (hermaphrodite, gynomonoecious, monoecious) and their spatial pattern within the flower head in this plant family to develop an elegant means of testing this hypothesis. Their results are consistent with their expectations on the effect of resource limitation on the head, as determined by patterns of fruit size within the head, assuming that female fecundity is more strongly limited by resource availability than male function.
The authors took on a huge challenge in choosing to study the largest plant family (about 25 thousand species). Their sample was limited to only about a hundred species, but species selection was very careful, to ensure that the range of sex conditions and the available phylogenetic information were adequately represented. The analytical methods are robust and cast no doubt on the reported results. However, I can’t help but wonder what would happen if the antiselfing hypothesis was tested simultaneously. This would require self-incompatibility (SI) data for the species sample, as the presence of SI is usually invoked as a powerful antiselfing mechanism, rendering the unisexuality of flowers unnecessary. However, SI is variable and frequently lost in the sunflower family . I also wonder to what extent the very specific architecture of flower heads imposes an idiosyncratic resource distribution that may have fixed these sexual systems in species and lineages of the family. Although not approached in this study, intraspecific variation seems to be low. It would be very interesting to use similar approaches in other plant groups in which inflorescence architecture is lax and resource distribution may differ. A whole-plant approach might be required, rather than investigations of single inflorescences as in this study. This study has no flaws, but instead paves the way for further testing of a long-standing dual hypothesis, probably with different outcomes in different ecological and evolutionary settings. In the end, sex is not only in the head.
 Darwin, C. (1877). The different forms of flowers on plants of the same species. John Murray.
 Barrett, S. C. H., and Harder, L. D. (2006). David G. Lloyd and the evolution of floral biology: from natural history to strategic analysis. In L.D. Harder, L. D., and Barrett, S. C. H. (eds) Ecology and Evolution of Flowers. OUP, Oxford. Pp 1-21.
 Geber, M. A., Dawson, T. E., and Delph, L. F. (eds) (1999). Gender and sexual dimorphism in flowering plants. Springer, Berlin.
 Charlesworth, D. (1999). Theories of the evolution of dioecy. In Geber, M. A., Dawson T. E. and Delph L. F. (eds) (1999). Gender and sexual dimorphism in flowering plants. Springer, Berlin. Pp. 33-60.
 Friedman, J., and Barrett, S. C. (2008). A phylogenetic analysis of the evolution of wind pollination in the angiosperms. International Journal of Plant Sciences, 169(1), 49-58. doi: 10.1086/523365
 Renner, S. S. (2014). The relative and absolute frequencies of angiosperm sexual systems: dioecy, monoecy, gynodioecy, and an updated online database. American Journal of botany, 101(10), 1588-1596. doi: 10.3732/ajb.1400196
 Torices, R., Afonso, A., Anderberg, A. A., Gómez, J. M., and Méndez, M. (2019). Architectural traits constrain the evolution of unisexual flowers and sexual segregation within inflorescences: an interspecific approach. bioRxiv, 356147, ver. 3 peer-reviewed and recommended by PCI Evol Biol. doi: 10.1101/356147
 Diggle, P. K., and Miller, J. S. (2013). Developmental plasticity, genetic assimilation, and the evolutionary diversification of sexual expression in Solanum. American journal of botany, 100(6), 1050-1060. doi: 10.3732/ajb.1200647
 Ferrer, M. M., and Good‐Avila, S. V. (2007). Macrophylogenetic analyses of the gain and loss of self‐incompatibility in the Asteraceae. New Phytologist, 173(2), 401-414. doi: 10.1111/j.1469-8137.2006.01905.x
Juan Arroyo (2018) Sometimes, sex is in the head. Peer Community in Evolutionary Biology, 100069. 10.24072/pci.evolbiol.100069
Reviewed by anonymous reviewer, 2018-11-11 23:55
Revision round #12018-11-08
Decision round #1
Dear Dr. Torices,
As a member of PCI in Evolutionary Biology I was asked to manage your preprint in order to get it recommended in this platform
“Architectural traits constrain the evolution 1 of unisexual flowers and sexual segregation within inflorescences: an interspecific approach”
Three competent colleagues made insightful reviews from different, albeit complementary fields, and their comments are available to you. I consider, like reviewers, that the preprint has a high value to be recommended by PCI Evol Biol, but at the same time, I consider that many of the comments and concerns raised by the reviewers are worth to be taken into account and responded properly. These reviews, albeit anonymous, and your responses will be posted in PCI Evol Biol portal. Since the reviews are made from different perspectives, I strongly believe that considering them in your response and/or revised manuscript will greatly enhance its quality and the readership of the final preprint.
I also would add some specific minor comments from my own side:
1. Given the very particular nature of the inflorescence type in your study, I would consider including some indication in the title. It might be adding “...in Asteraceae”, “flower heads”, “compact inflorescences”, etc. As the authors mention in their Discussion, it remains to be proved what is the case in other kind of inflorescences, specially those being sparse.
2. Please, consider to do an extra review of the English, I found some typos (e.g. line 76 “later”) and format of the reference list (some journal titles are no italisiced, similarly to some species names, among others). Reference by Funk et al. 2009 is published by IAPT?. Please, consider that a preprint is a publically available text and that PCI Evol Biol does not support proof correction.
3. Line 134. I have some doubt about the meaning of “phylogenetic gradient”.
4. Lines 186-188. A sample of 97 species might be low or high enough depending on its representativeness. It would be important to add more information in Table S1, for example adding the subfamilies, and the rough distribution of species. For those readers non-familiar with systematics of Asteraceae, a supplementary figure with the position of the sampled species in the Asteraceae tree would be very useful. I also wonder why four genera were represented by more than one species (particularly Vernonia). Did you think that there were within genus variability?, but this is not discussed in the manuscript. Is the tree used including within genus variability?
5. Line 374. Although I like the arguments raised by the authors, I consider that there is a possibility to test if geitonogamy (i.e. selfing) avoidance (as stated by Harder & Barrett 1995 considering pollinator movements) is involved by correlating SI data (as from Ferrer et al. 20007, cited by the authors) with your data, if possible. Incidentally, do you have wind-pollinated species in your data set? this information could be also useful in your Table S1.
Ferrer, M. M., and S. V Good-Avila. 2007. Macrophylogenetic analyses of the gain and loss of self-incompatibility in the Asteraceae. New Phytol. 173:401–14.
Funk, V. A., A. Susanna, T. Stuessy, and R. J. Bayer, eds. 2009. Systematics, evolution, and biogeography of Compositae. International Association for Plant Taxonomy, Viena.
Harder, L. D., and S. C. H. Barrett. 1995. Mating cost of large floral displays in hermaphrodite plants. Nature 373:512–515.