Three sex phenotypes in a haploid algal species give insights into the evolutionary transition to a self-compatible mating system

Balancing selfing and outcrossing: the genetics and cell biology of nematodes with three sexual morphs

Trioecy, a rare reproductive system where hermaphrodites, females, and males coexist, is found in certain algae, plants, and animals. Though it has evolved independently multiple times, its rarity suggests it may be an unstable or transitory evolutionary strategy. In the well-studied Caenorhabditis elegans, attempts to engineer a trioecious strain have reverted to the hermaphrodite/male system, reinforcing this view. However, these studies did not consider the sex-determination systems of naturally stable trioecious species. The discovery of free-living nematodes of the Auanema genus, which have naturally stable trioecy, provides an opportunity to study these systems. In Auanema, females produce only oocytes, while hermaphrodites produce both oocytes and sperm for self-fertilization. Crosses between males and females primarily produce daughters (XX hermaphrodites and females), while male-hermaphrodite crosses result in sons only. These skewed sex ratios are due to X-chromosome drive during spermatogenesis, where males produce only X-bearing sperm through asymmetric cell division. The stability of trioecy in Auanema is influenced by maternal control over sex determination and environmental cues. These factors offer insights into the genetic and environmental dynamics that maintain trioecy, potentially explaining its evolutionary stability in certain species.

Chlamydomonas reinhardtii, Volvox carteri and related green algae accumulate ketocarotenoids not in vegetative cells but in zygospores

Zygospores of green alga such as Chlamydomonas reinhardtii, Volvox carteri or Dunaliella salina display a bright orange color indicative of carotenoids, yet there have been no reports on their pigment composition. The genomes of these algae contain genes for homologs of the β-carotene ketolase (BKT) from the well-known astaxanthin producer Haematococcus pluvialis, that were assumed to be pseudogenes, because none of these species has been reported to accumulate astaxanthin or other ketocarotenoids. Here, we show that C. reinhardtii and V. carteri synthesize ketocarotenoids specifically in zygospores. Contrary to the vegetative aplanospores of H. pluvialis, the major ketocarotenoid in zygospores of C. reinhardtii is not astaxanthin but 4-ketolutein. Moreover, the ketocarotenoids in maturing zygospores are not synthesized de novo but from carotenoids of the photosynthetic apparatus liberated by a massive breakdown of thylakoid membranes. In line with this conclusion, incubation of zygospores at 9°C instead of 22°C resulted in a reduced thylakoid breakdown and only low amounts of ketocarotenoids, while the accumulation of storage lipids was less affected. Furthermore, we show the full-length BKT from C. reinhardtii to catalyze the ketolation of both α-carotene and lutein in carotenogenic bacteria. We also detected putative BKT genes in the genomes of various other green algae not yet known to synthesize ketocarotenoids, suggesting a zygospore-specific accumulation of ketocarotenoids to be common among Chlamydomonadales. Our observation that zygospores of C. reinhardtii accumulate ketocarotenoids together with storage lipids sheds light on the physiology of a largely unexplored algal life stage crucial for survival and propagation.

Swimming ability and flagellar motility of sperm packets of the volvocine green alga Pleodorina starrii

Eukaryotic flagella collectively form metachronal waves that facilitate the ability to cause flow or swim. Among such flagellated and planktonic swimmers, large volvocine genera such as Eudorina, Pleodorina and Volvox form bundles of small male gametes (sperm) called "sperm packets" for sexual reproduction. Although these sperm packets reportedly have flagella and the ability to swim, previous studies on volvocine motility have focused on asexual forms and the swimming characteristics of sperm packets remain unknown. However, it is important to quantify the motility of sperm packets and sperm in order to gain insights into the significance of motility in the sexual reproduction of planktonic algae. In this study, we quantitatively described the behavior of three flagellated forms of a male strain of Pleodorina starrii-asexual colonies, sperm packets, and single dissociated sperm-with emphasis on comparison of the two multicellular forms. Despite being smaller, sperm packets swam approximately 1.4 times faster than the asexual colonies of the same male strain. Body length was approximately 0.5 times smaller in the sperm packets than in asexual colonies. The flagella from sperm packets and asexual colonies showed asymmetric waveforms, whereas those from dissociated single sperm showed symmetric waveforms, suggesting the presence of a switching mechanism between sperm packets and dissociated sperm. Flagella from sperm packets were approximately 0.5 times shorter and had a beat period approximately twice as long as those from asexual colonies. The flagella of sperm packets were densely distributed over the anterior part of the body, whereas the flagella of asexual colonies were sparse and evenly distributed. The distribution of flagella, but not the number of flagella, appear to illustrate a significant difference in the speeds of sperm packets and asexual colonies. Our findings reveal novel aspects of the regulation of eukaryotic flagella and shed light on the role of flagellar motility in sexual reproduction of planktonic algae.

Whole-genome sequencing analysis of volvocine green algae reveals the molecular genetic basis for the diversity and evolution of sex

This review describes the development of evolutionary studies of sex based on the volvocine lineage of green algae, which was facilitated by whole-genome analyses of both model and non-model species. Volvocine algae, which include Chlamydomonas and Volvox species, have long been considered a model group for experimental studies investigating the evolution of sex. Thus, whole-genomic information on the sex-determining regions of volvocine algal sex chromosomes has been sought to elucidate the molecular genetic basis of sex evolution. By 2010, whole genomes were published for two model species in this group, Chlamydomonas reinhardtii and Volvox carteri. Recent improvements in sequencing technology, particularly next-generation sequencing, allowed our studies to obtain complete genomes for non-model, but evolutionary important, volvocine algal species. These genomes have provided critical details about sex-determining regions that will contribute to our understanding of the diversity and evolution of sex.

Reorganization of the ancestral sex-determining regions during the evolution of trioecy in Pleodorina starrii

The coexistence of three sexual phenotypes (male, female and bisexual) in a single species, 'trioecy', is rarely found in diploid organisms such as flowering plants and invertebrates. However, trioecy in haploid organisms has only recently been reported in a green algal species, Pleodorina starrii. Here, we generated whole-genome data of the three sex phenotypes of P. starrii to reveal a reorganization of the ancestral sex-determining regions (SDRs) in the sex chromosomes: the male and bisexual phenotypes had the same "male SDR" with paralogous gene expansions of the male-determining gene MID, whereas the female phenotype had a "female SDR" with transposition of the female-specific gene FUS1 to autosomal regions. Although the male and bisexual sex phenotypes had the identical male SDR and harbored autosomal FUS1, MID and FUS1 expression during sexual reproduction differed between them. Thus, the coexistence of three sex phenotypes in P. starrii is possible.

Digest: Three sexes from two loci in one genome: A haploid alga expands the diversity of trioecious species

Multicellular eukaryotes exhibit a remarkable diversity of sexual systems; however, trioecy, the coexistence of male, female, and cosexual or hermaphrodite individuals in a single species, is remarkably rare. Takahashi et al. (2021) report the first known instance of trioecy in a haploid organism. In contrast to other known cases of trioecy, the authors report evidence for genetic control of all three sexes by two loci. These results complicate models for sexual system turnover and expand the known diversity of trioecy species in several ways.