The moss Syntrichia caninervis is found in dry areas in various parts of Eurasia and North America. This case study delves into some aspects of the behaviour of this species in the Mojave Desert of Nevada in the USA.
This species is dioicous, that is, with separate male and female plants. In the harsh desert conditions many of the plants rarely produce sexual organs (the antheridia and archegonia). Amongst the plants where sexual organs were seen female plants greatly outnumbered the male plants. At a high elevation study site (1494 metres) the ratio was 14 to 1. At another study site with an elevation of 750 metres the researchers found no male plants at all. The researchers noted similar extreme ratios in other dioicous Mojave Desert species in the genera Bryum, Didymodon and Syntrichia. Rarity of males seems a widespread feature in dioicous desert mosses and, while female-only populations are known, there are no known cases of a male-only population. Moreover there are even cases of species where only female plants have ever been found. Didymodon nevadensis, Syntrichia chisosa and Syntrichia bartramii are examples.
The researchers also found that sporophytes were rare, with almost all the female Syntrichia caninervis plants likely to go through their lives without ever producing a sporophyte.
Given the small sample sizes used to produce some of the percentages given below, you'll need to read them as a good guide to Syntrichia caninervis behaviour, rather than taking them at face value down to the last percent. There's more information about the statistical analyses (and their statistical significance) in the original papers given in the reference button at the end of this page.
Why are there so few Syntrichia caninervis males?
It is very likely that the female:male ratios in the Syntrichia caninervis plants without sexual organs would be similar to the ratios reported above for the plants where sexual organs were found. At first thought such heavily skewed populations are very unusual, since you could expect roughly as many females as males. The gametophytes are haploid, with each gametophyte having just a single set of chromosomes. Sex is chromosomally determined with each male plant carrying a single Y chromosome and each female a single X chromosome. In the process of sexual reproduction roughly equal numbers of male and female spores would be produced.
The gametophytes use up some resources in producing the sex organs so those organs are a cost to the plants. The male and female gametophytes have different sexual reproduction costs. For a male the cost of reproduction is simply the cost of producing the antheridia and the modified surrounding leaves (each such complex being called a perigonium). Once these have been produced there is no more expenditure of resources by the male. For a female there are two possible costs to measure. First there is the cost of producing the archegonia and modified surrounding leaves (each such complex being called a perichaetium). We would also need to think of the cost a female pays in carrying the sporophyte through to maturity. There's more about that a little later.
The production of perigonia puts such a demand on a male's resources that there is little left over to devote to survival. If there are extra stresses, perhaps physical damage that demands immediate repair or a detrimental change in the weather, a male may not have enough in reserve to either repair the damage or to defend itself against the weather change. By contrast a female gametophyte uses a much smaller proportion of its resources in producing perichaetia and so has far more in reserve. On average the annual gametophyte biomass produced in a single growth interval is about 200 mg. A perichaetium is allocated 1% of this biomass, a perigonium 8%. As a consequence, up to the time of production of the sex organs, a female plant has, on average, a greater chance of survival.
Why are Syntrichia caninervis sporophytes rare?
In the production of sporophytes there are two steps to consider. First, the female needs to be fertilized by the male. Second, a fertilized female must be able support the sporophyte to maturity.
Fertilization needs water to carry sperm from the male to the female plant and water is rare in the Mojave Desert. In bryophytes, especially soil bryophytes, maximum possible fertilization distances are typically short. For a good chance of fertilization a male Syntrichia caninervis plant has to be within about a centimetre of a female plant. Given the high proportion of female plants noted above, it's fairly easy to see that most females will not be close enough to a male plant. Moreover, it has been estimated that perhaps only 10% of the Syntrichia plants produce sex organs during their lives.
If, despite those obstacles, a female plant is fertilized, it faces the further stress of bearing the sporophyte to maturity. In the previous section you saw that in producing the perigonia the males bear a much higher cost than do the females in producing perichaetia (8% vs 1% of annual biomass production). However, if a female gametophyte is fertilized and carries a sporophyte through to maturity the total cost to the female increases dramatically and greatly surpasses the male's cost. A mature sporophyte represents about 114% of the annual biomass production by a gametophyte.
A female gametophyte, with a sporophyte well on the way to maturity, will have given a large amount of its resources over to sexual reproduction. Such a female will have become far more vulnerable to extra stresses, but very few females need to bear the cost of the full reproductive cycle. Two reasons have already been given: very few plants ever produce sex organs and the scarcity of males means that very few females are ever fertilized. Furthermore, even on the rare occasions when fertilization happens, there's a high chance that the sporophyte will be aborted because of adverse conditions. The greatest demand for water is at the beginning of the elongation of the seta and the demand continues through capsule expansion. The majority of sporophyte abortions occur very early, before the seta begins to elongate. More than one archegonium can be fertilized on a single female plant. In theory, such a plant could produce several sporophytes. A plant with more than one sporophyte would be far more stressed than a plant with only one sporophyte.
Careful dissection of a female plant will show inflorescences fertilized in the current year or in past years and traces of past sporophytes. Thus it is possible to gain an idea of the fertilization and sporophyte history of a specific plant. The team examined individual female plants that, over their lifetimes, had produced more than one sporophyte. It was possible to group sporophytes by cohort (ie, sporophytes sharing the same year of fertilization). When an individual plant had sporophytes in the same cohort, the probability of an abortion was much higher than among sporophytes in different cohorts (88% vs 64%). If the cohorts are ignored it's still the case that the more sporophytes a plant had started the more likely it was to have aborted at some time. The abortion rates were: 55% for plants with a single sporophyte, 67% for plants with two sporophytes, 70% for plants with three sporophytes and 81% for plants with four sporophytes.
By aborting sporophytes early in their development the gametophytes save greatly on resources, especially water and nitrogen, which are often in short supply. Sporophytes aborted before elongation of the seta had averaged about 8% of the mass of mature sporophytes, indicating a considerable saving on resources. By aborting sporophytes in stressful conditions, the gametophytes have much better chances of surviving. In addition, a gametophyte that stays alive through a current period of adversity may produce a sporophyte in the future.
Given all the obstacles to sexual reproduction, it is clear that vegetative reproduction is the dominant reproductive method in Syntrichia caninervis in the Nevada desert. Specialized vegetative propagules such as gemmae are not known in this species and vegetative reproduction is by fragmentation of the gametophytes.
The influence of shrubs
Desert shrubs provide some protection to the areas beneath their canopies. For example, the shading provided by a shrub may lower soil temperatures and also allow moisture to remain a little longer under a shrub. You could expect that the micro-habitats under and near shrubs would benefit Syntrichia caninervis. So it turns out. In another survey the research team studied 778 Syntrichia caninervis plants, 11 of which were males, 290 were females and 477 showed no evidence of their sex.
The numbers, sizes and masses of the Syntrichia plants decreased steadily the further away they were from a shrub. Most plants grew within 5 centimetres of the shrub canopy line. The largest individuals were near or under the canopy line, the smallest 35-45 centimetres away from the canopy line. The average size of individuals under the canopy line was twice that of the ones furthest away.
Sex expression (ie the production of sexual organs) was greater in the shelter of shrubs. Sex expression could be measured in two ways, firstly as the percentage of individual plants that expressed sex and, secondly, by the number of inflorescences per plant. There was a marked drop in both these measures for plants growing more than 5 centimetres outside a shrub canopy line.
Female plants were found both within shrub canopy lines and in the inter-shrub gaps. The 11 male plants were found close to shrubs, but it is impossible to draw any sound conclusions from such a small sample. The team then looked at fertilized female plants as proxies for males. In the previous section you saw that fertilization distances in Syntrichia caninervis are short, about a centimetre. Apomixis (ie development of a sporophyte from an unfertilized egg) is unknown in Syntrichia caninervis. Therefore a fertilized female inflorescence indicates that a male plant is or has been very near. As noted earlier, careful observation makes it possible to gain an idea of the fertilization history of a particular plant and thereby some idea of the historical distribution of male plants. The team examined 530 female inflorescences, of which 487 were within a band ranging from 5 centimetres inside to 5 centimetres outside the shrub canopy line. The other 43 inflorescences were on plants up to 45 centimetres outside the canopy line. Of the 530 inflorescences about 63 had been fertilized at some time and all these were in the 5 centimetre inside-outside band just described. The males that had fertilized those females must have been similarly confined.