Big Creek Fieldtrip Image

Landels-Hill Big Creek Reserve, Big Sur Coastline
Monterey Co., CA – January 18, 2003

It gets dark early in January! We did find what we were seeking, the brooding chiton, Lepidochitona thomasi (Pilsbry, 1898). This chiton is patchy in its occurrence and largely restricted to the Big Sur coastline, so our trip was a success! Prof. Eernisse is conducting DNA- and enzyme-based population genetics research on this chiton, newly extending research that he completed for his dissertation at UC Santa Cruz (Eernisse, 1986; 1988). Specifically, he is testing the hypothesis that brooding is a pre-condition for the success of self-fertilization. Chitons, like other marine animals, can either free spawn their gametes or brood them until the offspring can effectively crawl away. In the case of chitons, a female spawns her eggs along side her foot, where they are fertilized by sperm in the seawater, and the female "broods" them by sitting on them until these embryos hatch as larvae, typically after about 12 days, and typically when the emerging larvae already have a well-developed foot for clinging to the substrate. If offspring can crawl away then they could potentially later mate with close relatives, so this will be expected to result in locally inbred populations, which can then become vulnerable to the spread of genes that promote self-fertilization.

Here is how the multi-part hypothesis works. In a nutshell, selfing should be an advantage, but it is rare, especially in the sea. Where it does occur, it is associated with brooding. Such selfers are rare within their group, but they do exist, in various groups that normally have separate sexes. The reason postulated is that selfers will normally be selected against because of inbreeding depression. The exception is when they happen to arise in a population that is already highly inbred, so that there is little additional "cost" due to selfing. Then selfing can gain at least a short term advantage relative to cross fertilization.

Confused? Here is a more detailed account.

Selfing would seem to be a big advantage. A selfer should be able to eliminate that costly habit of producing sons, instead producing essentially all daughters that are identical to mom, and the daughters will produce all daughters, and so on, so selfers would be expected to spread their genes more effectively than strict females, who at best only produce about half daughters. This is the famous "two-fold cost of sex" paradox: sexual cross-fertilization should cost twice as much as self-fertilization based on the above argument so selfers should progressively become more common, yet cross-fertilization is by far the more common strategy seen in animals, especially in marine animals. However, quite a few marine animals (e.g., barnacles, flatworms, sea squirts) are hermaphroditic, but normally only cross-fertilize. In fact these cross-fertilizing hermaphrodites seem to invest about equally in female and male function. If an animal evenly splits its gamete investment between egg and sperm production, then this is roughly equivalent to a female producing about half daughters and half sons. However, that is not the sort of hermaphrodite one typically finds associated with self fertilization. Instead, hermaphroditic selfers tend to "invest" most of their energy producing eggs, producing only enough sperm to fertilize its own eggs. If a selfer is mostly female, then it can come close to gaining a two-fold advantage relative to females that cross-fertilize. This is exactly what is seen in most organisms that regularly self, and also in land plants, where selfing plants typically invest mostly in ovules, and produce relatively little pollen. Hermaphroditic flowering plants are common, but most of these invest about equally in female and male function, and actually tend to have quite clever contraptions or behaviors to avoid self fertilization.

So selfing is rare, but does occur associated with brooding. Why? That brings us to comparing free spawners with brooders.

Most chitons free spawn their gametes and, for approximately one week, the developing larvae are obligate free swimmers in the plankton. Thus, it is highly unlikely that a free spawned chiton will end up settling down anywhere near their parents. Contrast this situation with, for example, land plants, or land snails, where offspring are often nearby to their parent(s) and where selfing is quite common.

Prof. Eernisse is proposing that selfing is not more common in the sea because selfed offspring normally suffer inbreeding depression, as Darwin first showed over a century ago. This means that selfed offspring are less viable than cross-fertilized offspring because the selfers have phenotypes compromized by the expression of deleterious recessive alleles, so natural selection will operate: selfers should be selected against.

All the approximately 800 worldwide species of free spawning chitons are thought to have separate sexes with obligate cross fertilization, and this situation also applies to most of the about 40 species of chitons that brood as well (with brooding most likely arising multiple times, judging from the truly scattered taxonomic distribution of brooding in chitons). As a brooding chiton that has separate sexes, L. thomasi is particularly well suited for a comparative study of this sort because its nearest relative to the north, L. fernaldi Eernisse, 1986, is also a brooder but individuals are hermaphroditic and predominantly self-fertilizing. Prof. Eernisse demonstrated that, unlike L. thomasi, essentially all adult L. fernald could produce healthy offspring when isolated in the lab, plus they had hermaphroditic gonads with mostly ovaries, not testes, and he also provided field and lab evidence that individual L. fernaldi did not cross fertilize, even when in close proximity to other members of its species. Only one other normally selfing chiton species is known, L. caverna Eernisse, 1986, a somewhat more distantly related species that, coincidently, has also been found at Dolan Creek by Prof. Eernisse but has never been found at Big Creek, three miles to the south. It is also a brooder. It was not located on this trip, but that could be due to the limited time we had to look before the sun set. L. thomasi is the important "test case" for this study because it is the sort of cross-fertilizing population that is expected to be potentially vulnerable to self fertilization, should it happen to arise by chance mutation. It is vulnerable if it is already inbred, because then it is likely that harmful recessive homozygous phenotypes already eliminated from the population over time due to the regular inbreeding. Then there should be little additional "inbreeding depression" cost resulting from selfing, should a hermaphroditic individual with no block to self-fertilization happen to be born in such a population. In order to document inbreeding in L. thomasi, it is necessary to show that populations of along its range have less variation within populations than between populations.

References

Eernisse, D. J. 1986. The genus Lepidochitona Gray, 1821 (Mollusca: Polyplacophora) in the northeastern Pacific Ocean (Oregonian and Californian Provinces). Zool. Verh. (Leiden) 228: 3-52.

Eernisse, D. J. 1988. Reproductive patterns in six species of Lepidochitona (Mollusca: Polyplacophora) from the Pacific Coast of North America. Biological Bulletin 174: 287-302.

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