8


Journal of Paleontology 91(1):1–11


Figure 5. Schematic illustrations of Wiartonella nodifera thalli showing different ontogenetic stages: (1) stage 1 thallus with only unbranched hairlike laterals; (2) early stage 2 thallus with branched laterals; (3) late stage 2 thallus for which all second-order lateral segments have been shed. For simplicity, only two laterals per whorl are shown. See text for details.


dasycladalean algae reproduce through holocarpy, whereby essentially all of the cytoplasm from various parts of the thallus (main axis and lateral segments) streams into the reproductive structures and is devoted to reproduction, the thallus subse- quently perishing. Detailed photographic documentation of this process is provided in Berger and Kaever (1992). Thus, for a given thallus, it is reasonable to infer that reproductive volume (R), defined as the collective volume of thallus regions utilized for gamete containment, will approximately equal cytoplasm volume (C). The latter, like reproductive volume, is a small subset of total interior thallus volume, in this case because most of the thallus interior in life is occupied by a large central vacuole, the cytoplasm forming only a thin film along the inner surface of the cell wall (Ngo et al., 2005) (Fig. 6.1). In basic outline this approach is similar to that used by Maksoud et al. (2014), but a key difference is that their study did not incorpo- rate the central vacuole in calculations. To evaluate whether the predicted relationship outlined above


holds true for dasycladalean algae, biophysical modeling was applied to the extant species Dasycladus vermicularis, Neomeris dumetosa,and Cymopolia vanbosseae. For each, cytoplasm volume for a pre-reproductive whorl, including the gametophores, was modeled from aspects of external thallus morphology follow- ing the method described in LoDuca and Behringer (2009), and reproductive volume for the same whorl was calculated as the total interior volume of the gametophore, modeled as a sphere, multi- plied by the number of gametophores developed on the whorl. Applying this approach to morphometric data for a single specimen of each species obtained fromimages in Berger and Kaever (1992) yielded C/R values for Dasycladus vermicularis, Neomeris dume- tosa,and Cymopolia vanbosseae of 1.4, 1.8, and 1.7, respectively


Figure 6. Models applied to investigate reproductive functional morphology for Wiartonella nodifera:(1) primary lateral in pre-reproductive state showing thin layer of cytoplasm (gray) along inner surface of cell wall and large central vacuole (white); (2) choristospore model, with node treated as a gametophore and filled with cytoplasm; (3) cladospore model, with entire primary lateral treated as a reproductive structure and filled with cytoplasm. Details of lateral morphology and cytoplasm thickness roughly to scale, but less than half of total length of primary lateral shown. R = reproductive volume; c.v. = central vacuole.


(Table 2).Note that for the latter species, one-third of the cytoplasm volume from an associated “sterile” whorlwas included in the total cytoplasm volume, since there are approximately three fertile whorls (i.e., whorls that develop gametophores) for every sterile whorl on the thallus of this species of Cymopolia (Berger and Kaever, 1992). Without cytoplasm input from the sterile whorls, each ofwhich yielded a calculated cytoplasmvolume roughly three times that of a fertile whorl, the C/R for this species falls to 0.8, meaning that more reproductive volume would be produced than could be supplied by cytoplasm. For D. vermicularis and N. dumetosa, the same analyses were performed utilizing morphometric data presented inMaksoud et al. (2014). Using data from this source, the C/R value calculated for N. dumetosa is comparable to that reported above (1.6 vs. 1.8; Table 2). For D. vermicularis, however, the resulting C/R value differed by a factor of two (0.67 vs. 1.4). To explore this discrepancy, we col- lected morphometric data from an image of D. vermicularis in Berger (2006, fig. 82), the source utilized byMaksoud et al. (2014). Our measurements agree with those of Maksoud et al. (2014, table 8) in all regards except for width values associatedwith the second- and third-order laterals, which were larger by a factor of about two (Table 2). Incorporating our revised width measurements resulted in a C/R value of 1.5 (Table 2). Collectively, the results provide compelling evidence that reproductive volume closely approx- imates cytoplasm volume for living dasycladalean algae, as pre- dicted on theoretical grounds. That the calculated C/R values are slightly above unity in each case is reasonable given that it is unlikely a thallus could streamall cytoplasm into the gametophores, a proposition supported by images in Berger and Kaever (1992) which show some remnant cytoplasm adhering to the inner cell walls of all reproductively mature specimens. In general, there is no reason to think that a similar C/R relationship should not apply to extinct taxa and, crucially, it is possible to apply this approach to well-preserved fossil specimens.


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