Back Garden Biology — or from little things big things grow (with respect to Paul Kelly and Kev Carmody)
On a sunny winter afternoon during a long Melbourne Covid lockdown, I found myself crawling about in the back garden looking for insects with my daughter as part of her home schooling and we chanced upon a liverwort growing between the brick and bluestone pavers (pictured below). I had always wanted to see this liverwort, Sphaerocarpos, as it was the genus in which sex chromosomes were first described in plants more than a century ago by Charles Allen in 1917. Our back garden discovery led to a number of questions, from the simple and straightforward to the more philosophical and sometimes weighty. Over the past five years these questions have been pursued by two PhD students (Shilpi Singh and Jonathan Levins) and three undergraduate students (Billie Arch, Fiona Bosilkovski and Brooke Cottle).
We first wondered whether there existed in GenBank data to examine the evolutionary history of sex chromosomes in liverworts. Surprisingly, although the data was not collected for this purpose (a recurring theme in genomic studies), Shilpi was able to demonstrate that liverwort sex chromosomes were ancient, dating back to before the age of fishes in the Silurian, making them one of the oldest known examples of eukaryotic sex chromosomes.
While many extant liverworts, and the presumed ancestral liverwort, have separate sexes (dioicy), hermaphrodites (monoicy) have evolved multiple independent times. Thus, we next pondered how does a dioicous species with highly dimorphic sex chromosomes evolve monoicy. Shilpi sequenced and analysed the genome of the monoicous aquatic liverwort, Ricciocarpos natans, that was originally isolated from a billabong in the ACT and demonstrated that aneuploidy in which both sex chromosomes are inherited followed by a catastrophic chromosome rearrangement of one of the sex chromosomes resulted in the evolution of this monoicous species. As we examined the genomics of additional liverwort lineages, it became obvious that transitions from separate sexes to hermaphrodites occur frequently, and that the genomic signature for such transitions was conspicuous as it followed the same pattern multiple independent times — reproducible evolution!
Why would species evolve monoicy? Many monoicous species occupy marginal, ephemeral habitats and rely on brief windows of time during which aquatic fertilisation can occur. Returning to the Sphaerocarpos in my back garden — how did it get there and establish a population? Sphaerocarpos is a winter annual, going through its life cycle in as little as a couple months. The population of Sphaerocarpos occupied the area from which I usually exit my car, and then ‘downstream’ towards the street and bluestone gutters — thus, I was the likely vector for dispersal of this non-native species. Sphaercarpos is unique in that, while being dioicous, its spores remain united and are dispersed as meiotic tetrads, with two female and two male spores distributed as a unit. this condition is hypothesized by some to be the ancestral condition in land plants, and while being a derived trait in Sphaerocarpos, facilitates colonization of new habitats. Thus, a single tetrad picked up by my shoe from another locale would have been sufficient to establish my back garden population. As an aside, all floras of Australia suggest that the species Sphaerocarpos texanus was introduced into Australia in the mid-20th century, possibly via grain shipments. However, analysis of our back garden Sphaerocarpos refines this to be Sphaerocarpos europaeus (rather than texanus) and suggesting Europe as the source of invasion.
Analysis of the genomics of our Sphaerocarpos europaeus isolate (primarily by the three undergraduate students) led to discovery that evolution from derived monoicy back to separate sexes occurs frequently, and that Sphaerocarpos represents one such case. Just as in the transition from dioicy to monoicy, the transition from monoicy back to dioicy also has a distinct reproducible genomic signature. That this has occurred multiple independent times supports the idea of strong selective pressure for eukaryotes to have sex. While we pondered this big question, we have not added to the copious literature surrounding the ‘whys’ of this topic, the multiple re-evolutions of separate sexes in liverworts reinforces the idea that for long term (geological scale) survival, sex is best.
Two genera in which we characterized the re-evolution of dioicy are Sphaerocarpos and Riccia — two genera that are also united by another phenotypic characteristic — reductive evolution (e.g. think cave fisheyes or cetacean limbs). Species of both genera have a reduced morphology compared to their ancestors and have lost the ability for spore dispersal, and instead spores are just retained in the parent plants until they decay. Both genera have also lost several transcription factors that regulate the morphology and anatomy required for spore dispersal. This led us to ponder whether during the transition to monoicy and back to dioicy again these species bought a one-way ticket to Palookaville in an evolutionary sense.
While our investigations have focused on a few ecologically insignificant liverworts, as we reach mid-2025 after the warmest year in human history, we are led to pondering whether climate change could lead to similar changes in angiosperms with a loss of insect pollinators? Is this the big picture of how genomic evolution works — gradual degradation and loss in many lineages due to changing environmental conditions with only a few ‘intact’ lineages able to cope with the next mass extinction? Profound big questions arising from looking at tiny back garden plants, and while the future may be uncertain, the present (with the help of the Centre) has been full of the pleasure of finding things out (with respect to Richard Feynman).
John Bowman
Chief Investigator, Monash University Node Leader, and Researcher Development Leader
